Please tell me how many 400whp sr20s or ka-ts you know exist here on SON lol 1,2,3!!!while those v8/v10s are N/A they still flow more than any 4banger @400whp. you are right turbos do like flow and a smaller exhaust would be more of a restriction at high rpms, BUT its still not necessary for a street machine even @400whp… alot of guys have run 11s even 10s on 2.5 inch exhaust without any issues.(always remember the practicality factor when building a street car)
now if ur talking a full race/track car then its a different ballgame.go big 3inch 4inch whatever or take it off completely ur the man more power to you literally…