what size exhaust tube/elbow etc for my setup?

[CNHSS1]

Ok so my cut'n'shut DET manifolds now finished (see thread http://www.nissansilvia.co.uk/vbb/showthread.php?t=1466 ) so nearly ready to trial fit it to the car ready to make a new elbow, downpipe and exh system. My question is regarding the bore of tube to use. Bit of background on the spec and what its used for may help with your suggestions.
its a CA18ET as in the S12s with Autosprint cam, flowed head, KA t/b, CA20E inlet manifold running 444s, forgies and steel rods, with a turbo technics T28 lo-lag hybrid, all controlled by an Emerald aftremarket ecu. Its used for sprints and hillclimbs, so response is key rather than out and out power but that said dont want to restrict the motor if i wish to up the boost again
currently runs 250brake at 16psi but looking to run bit more possibly?....

current system is 63mm (2.5"), will that be enough or do i go 2.75 or 3"?

Ive got a elbow now (cheers Mike ) thatll be cut up for the flange and the lambda boss.