Stock/Unmodified Reedcage
Even on stock bikes I recommend replacing the factory steel reeds with fiber reeds. With the money you spend when you have to rebuild one of these engines to me worrying if one of those steel petals is going to break off and ruin my engine just isn’t even worth it.

So there’s “basically” three options:
-Some guys like using YZ125 petals part # 55Y-13613-00-00 because the reedcage they come off of is a 6 petal cage, they come as three individual petal joined at the bottom, so you buy 4 of these and cut one petal off of each reed.
- I like to use YZ85 reed petals part # 4ES-13613-00-00 they are a single petal so you buy 4, they bolt right on but you do have to trim the front edges to narrow them just a bit at the front easy peasy.
-Here in the US TDR makes a racing petal for the RD’s, they are a single petal reed that bolt right on but are a little thicker than stock YZ85 or YZ125 petals so they bias topend performace a bit.
(There are of course other brands and materials but these are 3 great options)

The next step up would be:
Stock Modified Reedcages
For racing to move more air through these engines at some point you just can’t afford not to at least modify the stock reedcages and some vintage racing organizations require that you retain the stock (although modified) reedcages.

Modifying RD/RZ Reedcages
The reedcage is one of the easiest to modify and first attacked pieces of a 2stroke. But remember this: From the tip of the air cleaner to the tip of the silencer your 2stroke is a series of varying sized and shaped passages dealing with varying rpm, load and harmonics so increasing the size (or shape) of any one point will only show a gain if that point was or has become a restriction. 
With that said this is what I've found/learned to work on RD/RZ reedcages.

2 Strokes need to move alot of air through the reedvalve quickly. The easiest way to get the most air soonest (the quickly part) is to have alot of reed opening area at the very tip of the reedcage so that theres higher flow as soon as the reed opens (lower reed lifts).Since the majority of the action in a reedcage is at or starts at the tip  thats were I start. more airflow at lower reed lift is accomplished by moving the reed opening forward to within a mm or 2 of the tip this not only makes the reed opening longer but makes "A" thinner less of an obstruction. blend & shape towards the back "B" but do not knife edge, knife edges are for knifes not for 2strokes. Next smooth the sides "C" for a little more width & less turbulence but leave at least a mm for sealing. Next narrow the bridge, 4mm is fairly safe, 3 less safe.  Modifying the back of the reed opening "D" has the least effect due to the low reed lift there but is sometimes necessary. I use small fiber cut-off wheels & standard burrs for the rough work & an assortment of files & sandpaper for finishing.

I’ve never had the opportunity to experiment with the the angle of the horizontal divider but Tony Doukas of Tony Doukas Racing had this to say about it. “By changing the angle of the center divider that splits the cage top and bottom (The horizontal divider) you can alter the airflow so it pushes the reed petal at different angles. A steeper angle will give more bottom response but affect flow at high engine speeds.  A shallow angle will have less bottom end and create more flow at higher RPM's.  This is the normal angle we use as the Banshee engine it a mid to top end engine.”

In higher output RD400s removing the bridge & using single petal YZ85 reeds or  "TDR" (Biases top end) works great. 

The next step up would be:
YZ85 Reedcages
These supply a substantial increase in reedtip area and are single petal cages right out of the box. I use these as often as I can because they are a large improvement in reed area, very affordable, they don’t require as much thinning of the intake port to get them in as any of the RZ/Banshee reedcage options do (which risks breaking through the casting), and because they are a rubber coated cage the reeds last a long time. I also like them because unlike using RZ/Banshee manifold with RD cages where you have to cut off the manifolds “ears” which are actually airflow guides.. You can narrow the RZ/Banshee manifold “ears” by 2mm on each side with a right angle sanding tool and they slip right in so you keep the guides and get the nice RZ/Banshee crossover ports to boot.

The next step up would be:
Moto Tassinari V-force YZ85 Reedcages
These reedcages provide even more airflow over the Stock YZ85 reedcages.

RZ Reedcages
I haven’t dyno’d RZ350 Reedcages against YZ85 Moto Tassinari cages yet but my seat of the pants tells me that The YZ85 Moto Tassinari cages will out perform the of RZ350 Reed cages.

Modified RZ Reedcages
Why am I not saying RZ/Banshee Reedcages? Because the front edge of an RZ350 reedcage is just a little bit thinnner than a Banshee reedcage allowing you to get just a little bit more aiflow out of the RZ reedcage. 

On RZ350 reedcages because theirs not much airflow at the rear of the reedcage opening just square off the openings and concentrate on narrowing the bridge and working on the front and sides of the reed openings increasing the airflow where it's most effective

Removing the bridge will work on the RZ cage but I don't unless it's  absolutely necessary as it can cause a bit of an upset at lower rpms. I don't recommend it, but if you have to don't move the reed openings back or square off the outer rear corners of the reed openings. This will give you about as much support as you can get without the bridge.

Moto Tassinari V-force Banshee Reedcages
It takes some work to get them into an RD and honestly are overkill for most RD’s but they have handled up to 120hp in Banshee engines.

TZ750 - RZ350 Reedcages

I often hear people wanting to put TZ750 reedcages in an RD or RZ but as you can see from the picture the TZ750 & RZ350 reedcages have exactly the same actual "reedcage" area & thus flow rate. TZ cages were used because at the time RZ cages weren't around yet. TZ cages required either welding an adapter plate to the cyl or redrilling the mounting holes. RZ cages have the right mounting holes already & are slightly easier to fit in an RD because of the trimmed off edges at the base of the cage.

RZ Intake manifolds with RD reedcages

A good intake modification for the RDs is to use RZ350 intake manifolds with stock modified RD reedcages which allows you to use a boost bottle or crossover tube & still retain the stock reedcage. Since the bolt pattern is the same all you need to do is cut the 'ears' off the RZ manifolds that would fit into the RZ reedcage flush. Then put the manifold up against the RD reedcage & mark the difference in the openings on the cage, remove the manifold & cut there to match the two.

ReedCage Spacers

Spacers between the reedcage & the cylinder can be beneficial & are used to 1) unshroud the boost port when the airpath is obstructed or poor by design or when necessary when the boost port has been modified 2) Used to help in fitting a larger reedcage or 3) To increase crankcase volume.

Installing RZ reedcages into an RD

This is a common performance mod but 2 things first. 1) do you really need  it? I've personally put 70 rearwheel roadracing hp through modified RD reedcages. 2) It's time consuming. If you decide to do it be careful & take your time & you can do this a hundred times or rush & you'll hit daylight. Start by putting dykem on the face of the cylinders reed gasket surface & using the mounting hole threads as a guide mark as shown those limits have personally never let me down. now put the spacer you'll be using on the reedcage & measure from the face of the spacer to the tip of the cage measure that far into the intake & mark it 'all the way around' that way if you wipe out the marks on the sides you can still judge by the top & bottom marks. Then measure the reedcage tip & set an inside divider to that width, you'll port from the marks at the sides of the intake gasket surface to that width at the other marks you made inside the intake port. Now blend it in up to the liner. Then comes the fitting. 
Using 2 long points cover bolts without reeds or reed stops on slide the cage with spacer in as far as it will go tap it a bit, remove & port were it marks the port. Repeat till it fits then put on the reeds & guides & repeat until it fits. Do a final cleanup & make sure the reeds don't hang up when fully opened.