id say 250 ish rwkw using std pistons any more id be looking at forged...
Remember just cos their forged doesnt make pistons indestructible. Just means they can take a little bit more of abuse (bad batch of fuel, pinging) before they let go....
Rb30det's make power the same as other rb motors...it all depends on the support systems and how much $$$ you wish to spend.
Important thing to always remember is speed costs money...how fast do you want to go? Id budget for 10k + to even get over the 300rwkw mark.
BUT it can be done on the cheap...stick to 250rwkw and you can get away with a relatively reasonable budget. Use std bottom end (only get rings, hone and new bearings), buy head & slap it on, bingo budget motor