Hi
What about a T28 from the S15 which is a real BB with metal rather than ceramic wheel and supposedly gives a nice increase and can support up to 18psi without a real problem?
I was running the Rb25 turbo but as you said its still ceramic so its still limited to how much boost you can run.
I have seen the RB20 turbos upgraded with T3 metal wheels which claim decent HP for cost but it was still more expensive then buying the S15 T28.
Rob