2 1/4 would be as much as you would want to go with your engines guys. Thickness is not the right term to use either when speaking of exhaust sizing. Not to be a nitpicker, but its piping diameter. Thickness of the piping would imply having to do with heat properties.
The 2 1/4 piping is adequate to give good back pressure, which is what will keep you from having loss of torque. Anything more and expect to lose a little down low. If you go turbo, then you can think about 2.5" piping because you want to let the turbo breathe as freely as possible.
The gains you guys will find from exhaust mods will only come after getting rid of that cat in your exhaust manifold though!! Good luck!!