I heard as low as 15% and as high as 29%. I can tell you that Martin from AMS did back to back Dyno's with an Evo. On a FWD dyno they made 620whp and on the AWD dyno it made 605whp. That is only a 2.5% difference when he thought it would be a hell of alot more.

Personally, with FWD cars at about the same power level when i had my 16g, i would get slaughtered from a roll. They were 1g FWD vehicles though so you also need to take in account the weight. Traction will not be your enemy if you get LSD and a nice set of slicks. I know a guy on our DSMlink board who took he FWD 2g into the 10s on a t-67 turbo @35psi. He even launches at 6500 rpm...lol.