There are plenty of unofficial records and leader boards for typing speeds where qwerty typists are just as capable (more so even due to sheer availability) as Dvorak typists at hitting 200+wpm, and while that alone shouldn't be used to judge the quality of a particular layout it shouldn't be ignored that there's no difference in attainable max speeds done in short bursts. That says nothing about sustained typing of course, but I strongly suspect if you're already a competent and fast typist in qwerty you'll be comparably as fast in Dvorak and vice versa.
[1]: http://www.recordholdersrepublic.co.uk/world-record-holders/...