My understand of things is a little different from most. I believe all clutches are pretty close to the same on the low end (except for the heat generated) it's on the top end where they're different.
Pete Mueller had an article on this subject, which explains it much better than I can. Still, notice how the direct drive engine turns more RPM than the clutch drive! Some of that is due to the fact that they run lower gear ratios, but it's my conclusion they also get more power to the axle. I have to believe it takes some power to do the things the clutch does. It's got to take some power to sling those shoes out against the drum. This means less power to the rear axle.
On the low end, where the clutch is slipping, heat is generated. It takes power to generate that heat, meaning less power goes to the rear axle. Less heat, more power to the axle. I have no idea if that generated heat has ever been quantified, it would be an interesting experiment.