The WRX should work fine if you can get the WRX OEM Up/Down Pipes to fit into your RS (or have custom one's made). I think that there are several people undertaking this type of project. As far as 5 psi, you would need to get some type of new wastegate, since the spring in the TD04 is set for something around 14 psi. The WRX turbo is probably a little bigger than the Legacy RHB5, but like the Legacy turbo, you will probably run out of steam in the upper RPM range.
get a manual boost controller and set it low and you can keep it around 5 psi. as for when you hit the upper range, the turbo will run out of steem so you won't have the top end like with a garrett t3/t4 or vf23.
The way I see it. If a TD04 from a WRX can flow enough air for 217hp on a WRX, it can flow enough air for 200+ hp on a 2.5. It is a little bigger than the RHB5, I've actually had both installed on my car, but have not run the TD04 since it's not mine.
If you go that route, I have some of the parts you will need on my site. You might also check my turbo parts list for recommended parts for the swap you are considering. A basic setup with this turbo will run around $1500.
I have one up pipe in stock for $200 if you are interested and can have a downpipe in about a week. I'd suggest Jet-Hot coating though.