I just finished putting the Flowmaster Force II system (#17217) on my LT4 Friday... There is absolutely NO RESONANCE at all in the system... It's a big improvement over the complaints I've heard about the older American Thunder system...
The only negative I've encountered was fitment... In order to get the left muffler to line up properly I had to bend the rear pipe using a jack... The rear section also touches the differential so I had to use some firesleeving in there as a temporary insulator until I can get to an exhasut shop and get it fixed or replaced with a new piece... I'm also on the Corvette Forum and ran across another Force II system owner who had the same problems with the rear section touching the differential... As long as you can handle the possible fitment difficulties or have a shop to do it for you, I'd say go for it... Myself, I spent abuot 1.5 days to get the pipes bent right and mufflers hung before I was satisfied...
Sound wise, it's louder than stock but not as loud as my previous F-Body with Flowmaster cat-back... It has a nice rumble at idle, sounds great running up to WOT, sounds good at cruise, and best of all.... It doesn't pop when you decellerate in gear like I'm used to with all the other systems I've seen...
Summit Racing has this system the cheapest I've seen, $537, free shipping, and no sales tax (for me)...