exhaust and suspension problem...
 

I just got a full 3" custom cat-back set up on my 2004 GT, it sounds great and the throttle response has definitely improved since the mod, however, I'm having one problem whenever I have someone riding in the backseat or even on decent sized drop-offs or speed bumps my pipes knock on the rear axle and make a loud clanging sound, my question is do I stiffen up the rear end shocks with coilovers or something, or do I suffer and back to 2.5" tubing???,thanks...

I would take it back to the place where you got the exhaust done and demand that they fix it so that it doesn't interfere with anything as the suspension travels.

What company should I use for a complete header to tip exhaust. Which would have the best gains I have a 04 gt manual trans. I want a deeper sound but not a loud exhaust. What kind of gains should I expect.

You get the best gains from longtube headers, but they are not smog legal and reduce your ground clearance. Which setup you should go with depends on your taste in exhaust sounds and what your plans are for your car. There are some sites that have sound clips of all different combos. I'd do a search and check them out.