I think you'll be pushing the standard high flow turbo pretty hard if your aiming for 250rwkw its doable, just how long do you want the turbo to last is the question. Definitely need to tune a/f ratios and timing aswell.
300rwkw (400hp) is alot of power for a standard RB25DET, so after youve done all the support systems ie fuel system, computer,suspension etc youll have to get the internals done $$$$$$$$$$$$$$. Which isnt cheap. Then you might want to upgrade those brakes.
I reckon aim for around 220rwkw with the highflow and the pfc, this should be safe and reliable.
Just my opinion