'04 GTO - Best Performance Increase ?
 

What would the best bang for the buck to increase the performance of an 04' GTO,
without compromising the integrity of the engine, or exhuast?

1) nitrous oxide kit (how much $$ to refill the bottles)
2) heads / cams pro performance package
3) cut out mufflers only and replace with straight pipes
4) reprogram factory computer settings with an SLP Predator

Best bang for the buck is definitely going to be nitrous. No other mod you've mentioned is going to add 150+ horses at anywhere near the cost of a nitrous kit.