If your just considering a catback or mufflers, I would just pick the one you think sounds the best as your not going to gain much if any performance. The muffler bypass mod (or Ti-mod as Z06 drivers call it) seems to give good sound and is much less $$ than a catback.
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99 GMC2500 EXTCAB 4x4 Long Bed 6.0/ homemade cold-air/duals/ ported TB/HPPIII
99 TransAm/M6/Lid/XS longtubes/LS6 intake/Borla/ported TB/FTRA/!air/!egr/!cags/Frost tune
01 TR Z06/MGW shifter/!cags/LG headers/VR airbridge+relocated MAF+S&B oval filter/ported TB/Frost tune | 
thoughts on exhaust....... 
