A "pure" aerobatic aircraft does not want to be fast - it wants drag to keep the speed under control on the down-lines. I would say gear is only part of the factor - the RV series of homebuilts are poor aerobatic trainers, in turn due to their low drag (and that's with fixed gear!) and how the speed builds up (to beyond Vne) very easily
Red Bull is different - they are racing, so yes, retractable might be an advantage, but I suspect their rules have something to say about it.
NoD