Dark Greens, i believe only flow enough fuel for around 22-23psi or so boost. So anything above 330-340bhp AT THE FLYWHEEL sounds like poo poo.
1.2bar (17psi) held, could be a healthy 290-310bhp AT THE FLYWHEEL i would guess.

Good luck with the RR, the main thing is that it runs bob on. You didn't mention a breather kit, if it's not got one, probably the first thing to get.

Dave