Jerry Miculek races a Slidefire stock and achieves about 430rpm (10 rounds in 1.39 seconds):
http://www.thetruthaboutguns.com/201...umpfire-stock/
The Slidefire stock won on speed. Miculek won on accuracy.
And actually, repealing the Hughes Amendment would kill bumpstocks dead and make sure their replacements were background checked and registered. I bet if you sold it that way, you'd have 90% of the gun control crowd signing on before they figured out the hook.