I am running GT-RS turbines on my stock RB26. My turbos would be a fair bit bigger than a single GT3540.
I was originally running standard cams with vernier cam gears. I found that the best response was acheived with 0 degree timing for both inlet and exhaust. With this setup, the best I could achieve was 1.3 bar at about 5500-5600RPM.
I then dropped in a set of PONCAM type B cams (still running at 0 degrees in and ex). I now have 1.4 bar at about 5200RPM.
With the cams, I have definately lost a bit of power under about 3500RPM, but 4500-5000RPM, seems to be a bit better and MUCH better from 5000-8000RPM.
I reckon just spend the money on the cams and don't bother with the gears.