Here's my personal setup
Running on 170mm Titanium Hi-Power muffler (largest muffler that HKS offers)
Custom welded the inlet from 3 inch to 2.5 inch
2.5 inch stainless steel pipings
Custom Y-pipe (on it's way)
Z1 test pipes
And believe me, it is a beast with just a 2.5 inch of piping and stock Y-pipe!