I like a 6 speed but at the same time I know that your never going to out drive an auto at the 1/4 mile. With that said, what does the stock auto hold for power and how much money has to be put into it to make it hold even more? I am sure a clutch will keep the 6 speed on par with HP but I am sure the transmission it's self has it's limits as well before you start to have issues.