On the GT2540, the decision you have is what power do you want and what is your use - street, track or drag...?
I have 2x GT2540 and had HP drop off at 7000 rpm. I use it for street / track. Pressure management showed that the 3.5" Blitz turbo back exhaust was restricted at the cat and muffler. The turbos are at full boost by 4250 rpm. But...I have full forged internals.
Stock internals, I agree with Morgs. Stock you can push 330 wKW with a pair of GT2530s. But you cannot run this power reliably for track use - you will run lean in cylinder 6 and even with a power FC you will get pre-det once the engine bay, induction and track temperature rises.
If you plan for forged internals, increase intercooler and radiator size and add an oil cooler I would go GT2540s. You will run max HP spread from 5500 - 8500 with a 4" turbo back exhaust.
HL