that was rwhp in my r34gtr....
we ran the same engine and gt rs turbos in my r32 race car with motec and made 550hp at all 4 at 27psi.....
the gt rs turbos hit very hard and hold the power for a long time...
the only down side is you need balance pipe exhaust manifolds to help with the low boost compressor surge..
would have been nice to run 32 psi with my 4 inch exhaust and 50 mm thicker intercooler...i would say come close to 600 at all 4 wheels no worries... or 630 to 650 rwhp...
if you dont mind a bit of low boost turbo shuffle there are not many low mounts that will make that power...
cheers...hope this helps and not start a turbo shit fight....