Mr Rb20
I'd stay away from high flows of the stock turbo, unless you only want another 15-20rwkw. A series one R33 turbo will give you a bit of a increase, but you can only run 14psi due to the turbine wheel being ceramic.
After a lot of thinking i ended up getting a HKS 2510 turbo second hand. My goal was to have 200rwkw, with as little boost as possible, and still have almost stock drivability, and the 2510 fits the bill pretty well.
With my current setup - stock engine, tube and fin front mount cooler, Bosch motorsport pump and re-mapped stock computer, it made 198rwkw with 1bar boost on a warm summers day. The best part is that it drives very close to stock, just has a heap more top end.
The best part is that the turbo is still running lowish boost (for a HKS turbo) and has the ability to run 1.5 bar and make around 220-230rwkw with a revised fuel system.
Bolt on HKS turbos are awesome in my opinion, I'd recommend one if you want to upgrade
Chris