HFC fitment...2.5" or 3"?

bcfromfl

I'd like to start moving ahead with my planned mods to my car, but am having difficulty getting an answer to a basic question. I'm starting first with the exhaust, because everything else seems to build off of that first. Next clutch, then s/c, fuel, and tune.

I'm fairly certain I'm going with Garageline's X/Mid-pipe combo (2.5" diameter), and having a shop (haven't found one yet) weld in the cats into the X-Pipe. Both mil-spec and GESI come in 2.5" and 3" sizes. Which one would give the best fit and weld strength? The 2.5" would be a butt-weld, while the 3" would slip over the X-pipe and perhaps allow for a stronger weld. Also, the 3" would let the exhaust gases slightly longer to pass over the metal element and convert more fuel, as well as perhaps offer less restriction overall.

Once I get all this put together, it will allow me to evaluate how the stock Nismo muffler sounds, and either revise my plan or keep as is.

Thanks!