Hi Just came across your thread while looking for suff on GTR cam compatability with Rb25det heads... They work in RB20's and aparently make more low end torque and improve response.. still looking for RB25 data tho.
Anway, I run the same turbo on my RB20DET in my gts4. GT25BB400 with a 0.64 turbine, 0.7 compressor, turbosmart 38mm external gate, 600x300x75 fmic, GTR 440cc injectors, 20lb boost, wolf 4plus ecu, msd dis4 ignition and 3 coil waste spark ignition, 044 pump and a few other things.
I get positive manifold pressure from just above 3000 rpm, 10-12lb by 4000 rpm depending on load and gear, and full boost by 4500ish. I say 4500ish in that at that rpm with full boost within a second or 2 your at redline already so its really hard to say at what RPM it came on at.
The turbo doesn't really start to sing till rpms are above 4000 though. Probably far from being a good match for a RB20, but I use antilag as well, and when its in its effiency range it just goes and goes. If you weren't afraid to rev to 8000 it would hapily go that far. I shift at 7500.