My RB25 HKS 2535, combo made 244rwkw 14psi stock inj and AFM running a PFC.
I then bought injectors, Z32 boost controller etc, no more peak power but a massive amount of midrange was gained from 18psi... The extra boost totally transformed the car!
That same turbo is now on a mates car running 250rwkw at 15psi (i think) with the same setup (ECU, Z32 etc)
I also have another mate with a HKS 2535 on an RB25 with a Wolf 3D, injectors etc and he is running 18psi and 252rwkw...
I dont think they will run much more than 250ish rwkw no matter how much boost they have but the midrange agin on them is amazing at high psi...