+1
I have an RB25 with a high flow, at around 300hp i am maxing out the standard ecu, fuel pump and injectors, from what i know the RB20 injectors and fuel pump are smaller than the RB25 so i cant see you making 300 - 350 on your standard items.
There are 2 gaurentee's in modifying cars:
1) Power costs money.
2) You get what you pay for.
You have to spend the money and lay the ground work before you can step up, if you dont have an exhaust with full 3" turbo back including cat and dump pipe then that should be your first upgrade and will give you more power just there, pod/panel filter is also a must, then try upping the boost, after that ecu, injectors and fuel pump are next on your shopping list.
But if you insist on just going a bigger turbo then the easiest and probably best bang for buck would be an RB25 high flow.