I gained 8 rwhp and 5 rwtq when I installed my magnaflow catback on my 07 srt8 auto charger. But it was not stock, still pretty close though, all I had was a cai intake, 180 tstat, and a tune and got 381rwhp and 378rwtq, then later got a magnaflow catback and went back to the dyno to retune and got 389 rwhp and got 387 rwtq, so some gains. would have been happy with more since this kit runs $1,000.00+ for the stainless, but you can get it cheaper if you do your research or talk to some vendors on here, they will take care of you. But I will say that the stock srt8 exhaust does flow very well for being stock, I know of a few people that got almost the same amount of gains with just knocking out the reso's and changing mufflers out, thats why I went with the same on my 10' challenger, as far as knowing if it did the same.....who knows
I have not took it in to get tuned, just running a canned tune from my predator, from what it feels, it runs strong, pretty close if not the same to my 07 charger. guess the truth will come out when i dyno it.