The exhaust size is important, I have owned a number of turbo charged cars. it isn't important for the same reason mind you, well sorta not. Ok heres the deal with a small exhaust is restrictive, obviously, hence causing the turbo to spin slower. turbo lag. The reason a smaller exhaust on an NA car seems better is it creates a vacuum and scavanges the cylinders sort of sucking out the burnt fumes, but that only really helps at low rpms at higher rpms its another story. An engine is an air pump. the goal is to get air in and out as quick and smooth as possible, Honestly it all really depends on the type of driving you do or want to do smaller for a more stop and go type city driving bigger for a more racing type of highway, my starion has three inch from the downpipe back and it never had a problem. my xt has 2 from the turbo back with custom headers and a blown 12a (starion turbo) so i have no clue if i'll need to go bigger yet, i do know its loud as hell.