Dminor's right. The fastest way downhill is to carry the most momentum/speed through the fastest line. The only nit I might pick is that I don't know what "artificial" limits are posed by a rigid singlespeed. Equipment choice dictates some degree of limits, no matter what you're using. Maybe my only problem is with the word artificial.

Anyways, on trails I ride a lot I think it's more fun to be underbiked than overbiked, because otherwise I get bored. This may not be true for everyone. Hasn't led me to get a rigid singlespeed yet, but I go through phases where I usually grab one of my cross bikes instead of my hardtail. I guess that's a different discussion though.