We've even had members (Mainly Tone, God Bless Him!) try to polish and go as far as ceramic coat his intake manifold to improve in-flow. I've seen ceramic coated exhaust, exhausts with minimal bends, duals set up different ways, different mufflers, different tips, different cats, no cats, just about anything you could imagine, it's been done. Results were verified by actual and 1/4 times with computations while being hooked to equipment dyno not just by how it feels or the "Butt Dyno" and guess what, Isuzu got it right. There was very little improvement without major sacrifice in economy.
The best power improvement was/is (if you ever find a kit again) the Supercharger. But again, the economy goes out the door.