i like the design, although maybe u should include an option for shorter intercooler piping, gtr style.
plenums like this do make alot of difference. me and my mate tested the difference between our cars, both stock rb25's, mine had custom plenum, same size turbo. mine would kick in at 2200 and hit 1 bar by 3500 and his would kick in at around 3500 and hit 1 bar by 4500 rpm. although his was running stock computer with a safc and i was running a microtech LT-16s, im sure that tuning didnt make that much of a difference.
keep up the good work John