As for performance and results from the spacer. I havent heard alot from H2 owners yet, but from other vehicles I have. This spacer is the exact spacer you bolt on to any newer chevy 4.8L,5.3L,6.0L and the 8.1L. We have installed quite a few on the silverados and have notice a good difference in some and others not. We took a 02 Silverado 5.3L and installed the spacer, all it had was a cat-back. Well ran it a few times in the 1/4 mile before the spacer and a few times after the spacer was installed. Believe it or not, it went 4mph faster in the 1/4 mile with the spacer!
Every sigle time too. So we were really impressed! As for gas mileage, the ppl that have noticed it are on long trips and some ppl have noticed a slight difference. I think for the price of $75.00 we sell these for it worth it to put it on and give it a shot.