i have also heard that. there is no power difference between a 4" and a 5" however you get a better lower grunt sound with the five, so it might be worth the extra $ for the sound but you wnt get any extra hp. as for cost it depends on what kind of exhaust you want duels? stacks? single?
01-24v 2"level long box...stock for now 120k...NO SMOKE NO FUN!!!!