You'll get the more out of the tune with a CAI and cat-back. I haven't seen any real numbers supporting HP improvement with axle-backs, except maybe the Borla.
I'm interested in seeing what the new GT350 has to offer. Depending on the source, estimates are in the 550+ hp range but no price. I would seriously consider that over the the Hennessey, Roush, or Saleen. I would sell my '11 GT for one of those.