Hard to say, you need to find out what the max. efficiency of the 2540 is.
It has an A/R of 0.64.
Max. pressure is something like 2.0 or 2.5 bar AFAIK.
I don't think 28 psi is advisable with stock internals
I would think a 18-20-22-24 psi would be fine.
I would use Andrew's car as a guage.
With a GT3040 and 1.2 bar it hit 280rwkW but that's with Z32 AFM, injectors, manifold, all the high fuel bits...
250rwkW might take 22+ psi in all likelihood.
T.