Yeah..a little bit of help in the MPG department but I couldn't really tell any in the "power" department. The biggest gains were in tonal quality and overall noise REDUCTION. My glasspacks were so bad that it was almost unbareable. I remember pulling up to drive thru's (back when my K5 would fit thru them!!!) and the drive thru schmucks asking me to turn off my truck because they couldn't hear me....I'd laugh my ass off at them first, and then turn it off. Now with the flowmasters, dual pipes straight from the manifolds (yep..no headers), no cat's and a crossover pipe it just sounds SWEET!!! Nothing like the purr of a nice 350!!!! Good luck with whatever you decide.
CHEERS