I don't want to hijack MPFabs thread but just had to comment on this.
I don't know how the calculations work for how much power you can get out of what size exhaust system but I have a 1992 Mazda MX5 with custom turbo set up on it. It has a full 2.5" exhaust system from turbo back, I have the 2.5" seperated gases down pipe from BEGi hooked up to a Gt2060 & it makes 201kw (374hp).