ARH headers are nearly $1500 without cats for my 2007 5.3L.. An awful lot of cash for 20hp.. Stainless Works are $920 - a bit more tolerable but still not much gain for a ton of money. I think I'd be better off trading for a Duramax.
I want more power for towing only.. A bolt-on supercharger seems like an easy way (but you'd probably want to do exhaust anyway)..
Choices, choices..