The HRC downpipe is only 2 1/4 I believe, so how does 3" help further down the pipe? Is 3" catback pipe really worth it, or even needed? I dont plan to go above stage II anytime soon, so would 2 1/2 be enough?

From what I was told 2.5 should be fine. I think the numbers was it was able to support 250-300 horses. 3" I think would awesome for spool-up time but I don't think it would be practicle for anytbody who doesn't have built internals.