I have one on my 68 chevelle. I went from 2" duals all the way to the rear bumper with glass packs to 2 1/2", cross over tube, flowmasters and they dump out at the rear end. WOW! The difference is unbelievable! Of course, a lot of it is the new mufflers and bigger pipes, but it sounds so much cleaner and a lot more free flowig at idle. And when I stomp it, LOOK OUT! I would reccomend one for sure. Really cleans things up and helps with performance. unfortunately, I dont have any actual numbers to pass on, but I can just "feel" the improvement....all the way from idle to top end. I hope this helps ya out man.
--Jerry--