I've got an '04 Chev Avalanche with the 5.3L engine and have done a number of things to it because my AS is right at the limit of what the Avvy is rated to tow ... I'm trying to buy some time until I can buy one of the new diesel TVs that are supposed to be a year or two away. Anyway, I've installed:
1) tranny oil cooler;
2) Diablo predator programmer;
3) Corsa cat-back stainless steel exhaust;
4) Brute force air induction kit.
If there is a mileage benefit, it is negligible. It definitely sounds better (i.e. "badder") without being too loud b/c I chose the Corsa "touring" exhaust which is quieter than other exhaust systems. It definitely has more power in the mid- to upper-RPM ranges. When I hit about 2500 RPM it really takes off. However, at lower RPMs and towing I haven't noticed a great difference, so I don't think the torque benefit is significant. Bottom line I would say (i) horsepower/fun factor - UP; (ii) torque - Not much change; (iii) gas mileage - Not much change.
'06 28' Safari SE / '08 Chev 2500HD Duramax
'64 Me / '60's-ish wife / '90's kids / '06 Black Lab / '02 Poodle / oh, and a horse, too.