I have worked at a leading Sydney Nissan performance shop's for years and have built over 50 GTRs of this level, I have 5-10 private customers with this same engine combo producing 850+ with slightly smaller turbos on 35psi-38psi At the wheels, so its defiantly achievable.
I have seen standard head rb26 with T51R (garrett) go 865hp at fours on 34psi, so its easy to do the math, I would leave this engine at 850hp for reliability
any questions feel free to ask