No faster kill than a blunt and a head shot. Especially if you're in thick brush, I think it would be better for the arrow not to pass through. Every rabbit I've shot with a broadhead has run a surprising distance, which is why I don't use them any more. But the arrow sticking half way through will slow them down a good deal in thick brush. Hex heads will do that, and have pretty good shock value. I wish they were a little bigger though.
One of these days I'll get my dog trained up enough to heel quietly at my side, and only chase on command. I think that would be great bunny bowhunting. You and the dog sneaking around slowly, when you thump a bunny, send the dog to catch it immediately. If the bunny takes off, the dog can chase and track it and reduce the chance of it getting away if you've made a less-than-perfect shot. But now I'm in dream mode, so I'll sign off.