It is widely believed in Miata cyberspace forums that the drivetrain loses 26 hp. Not a % as such but a direct loss that doe not change when bhp changes. The proof was a test using an engine dyno v. chassis dyno on the same engine for both an NA version as well as a turbo FI version. Bothe cases lost about the same hp (I think within 2 hp or so). That being the case, the drivetrain loss as a % gets less on the same car as hp goes up. I am probably not remembering all the details but you get the drift.