A few months ago I pulled my Superchip out to experiment with mileage and performance. Results were summarized in this forum, if you search back.
Bottom line is the chip is never coming out again. Not sure about peak power gains, but at normal driving rpm the truck has far more power for passing, takes less throttle to maintain the same freeway speed, etc...
I got much better mileage with the chip, but there is no real $$ savings there as premium fuel was more expensive. The main gains are in performance. It's a BIG difference on my truck.
1999 XLT S/C, 4.2 V-6
Bilstein shocks, Steeda Rear Anti-sway bar, 2" rear drop.
Superchip, Airaid FIPK, Flex-a-Lite Black Magic electric fan