I dont know how the slp would sound on the GTP but I have slp loudmouth catback on my 02 mustang GT and it sounds incredible. 1 year now and a new england winter covered in snow and salt and not a spot of rust. Try looking on youtube for exhaust clips and see what sound you like best, then ask the poster about the quality issues like fitment and corrosion. It might be a lil footwork but it beats spending your hard earned money on something you aren't completly happy with. I personaly hate buying things twice.

I had an slp loudmouth on my z28 and it sounded god awful. The GTP catbacks are not the loudmouth series. They actually have real mufflers.

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1998 GTP , CAI box
Waiting for install: 3.4 pulley

I also plan on putting an exhaust on my GTP, i originally was just going to purchase the 3800SC Stage 3 Performance Package, but im not too fond of the sound an SLP provides. I'm pretty much set on getting the stage 2 package, and then buying a flowmasters or magnaflow catback exhaust seperatly.