2.5" exhaust vs stock exhaust

Your choking off your exhaust going from a 3" downpipe to the stock 2.25". I would absolutely go at least 2.5" preferably with mandrel bent piping. 3" would be even better especially if you plan on doing a lot more modding to your GTP. The larger piping will help take down your knock retard and that will help your engine run better. You should also feel a little more power freed up in the mid-upper rpm range.

I have ZZP full header setup with 2.5 catback, on a 2002 GS with new long block from Summit racing(their last one) it was actually overkill until I installed ZZP stage 2 IC and ZZP Billet Fuel rails and 3.0 pulley and a tune. That's when the exhaust really woke up. Installing the headers was time consuming but time is irrelevant when its the outcome that matters. Now its asking for more and I have already maxed out the stock #33 injectors. Waiting on ZZP to restock 42.5# injectors and another tune. Its a big relief to look at my scan guage and see no KR at all even over 100MPH in no time