I like the Heffner kit a lot. I've installed about 8 of them - just finshed one Friday night and I have another GT in the shop which will also get a Heffner (along with many other mods). Note that only the Heffner and the Borla have an option for the rear bumper delete kits. With the Borla, you simply get new (shorter) tips. With the Heffner, you specify what version you want when ordering. On the car I did Friday (non Forum Member), I also installed the Stillen CAT Delete pipes in front of the Heffner. The sound WAS different - I would say a little more refined than it is with the cats in place.
Either way the Heffner sounds very good. Do a search on Heffner and you will see the feedback from others as well.