I am actually lead to believe, that as far as efficiency they come out on top of the chain V's belt V's shaft for transmitting power to the back wheel.
But their limits are clearly seen on the drag strip, where many of the HD crowd dumped the belt for chain, and those who ran with belts risked major belt failure. I used to work at the drag strip, and the odd ego or 2 was crushed at the tree, with belt failure. But having said that, I never rode my HD like a wimp, and it was far from stock, and I never had a problem with the final belt drive. Just maybe I was a few cubic inches short having problems, but the guys running stroker engines stayed well away from final drive belts.