Look no further than Cars and Cameras on their race mower, with a 40 series, belt couldn't take the abuse and it all locked up. You end up burning the belt up more. But that's for CVTs, if your talking centrifugal clutch, then I'm not sure.
But if your talking like the torq-a-verter (20, 30, 40 series), they dont last very long behind high horsepower. For a 670, or any large V-twin, you want a Comet 780, or something from CVTech, Gauged CVT, etc. For higher horsepower applications it's going to get really expensive, but the parts hold up much better.