You pose a very interesting question. I've not run across many who have jumped right to the "Cam" swap upgrade. Those that have.....have gone "All In." Meaning they've done intakes and exhaust at the same time....perhaps even head porting. Having said that...top rwhp has varied from 470's to 520's. What one has to balance is....driveability. Ones that post the ultimate high numbers might not really be best suited for driving on the street. They tend to perform best at very high rpm....like 7000+.
I gained 30-40 rwhp with a FRPP 84.5mm TB, 47 lb injects ( Probably not adding anything until I switch to E 85 ), off road X pipe and Carbon Fiber Airaid cai with tuning from AED, 91 octane. I have a 2013 Boss....so a little different than your GT.....but just an example. My bolt-ons and tuning ran me @ $2200...with me searching for deals. I'm putting down mid 420's for rwhp...my peak hp is just above 7200 rpm and my redline is now 8250.
Best bang for your buck would be forced induction.....hands down....you'll see bigger gains per dollar spent.
Wayne
