4 inch downpipe to 3 inch midpipe(catless) to 2.5 muffler section seems to be the best setup IMO. Also has been dyno proven by the makers of this setup( Turbo XS). And if you go with a bigger turbo, all you need to do is replace the 2.5 muffler section to 3 inch type, but is not manditory. Depends on size of turbo.VallejoWRX said:I was told that with the stock turbo there it no difference with 2.75" and 3" piping is that true ???