Look here for what to use for lowering your car:http://www.grandprixforums.net/threa...r-Look-in-here!!
I would say stay away from the KYB AGX struts and just go with the GR-2/Excel G's, they are cheaper and honestly work just as well and for springs I think the ebay DNA springs aren't a huge drop like 1.5" or so. I'm sure someone else will jump in and probably be able to tell you more about lowering your car though and what to use.
As for the mufflers, you don't really have to change those for flow, changing your mufflers is more for sound than anything. For flow since you have a GT you could do a PLOG and downpipe and that will help or you could go straight to headers like the speed daddy's that most on here will use. Those are from ebay as well and usually around the $215-$250 range. Hope this helps some.
