if i was tall enough to consider this, i would look at what extraordinarily tall pro cycling athletes use. track, road and MTB. i figure they are willing to do just about anything to gain an advantage, so they and their professional trainers and coaches have probably already explored this, in detail.

OTOH, raising the BB 20mm ain't much... i wouldn't be surprised if the BB's on most cyclocross frames are not 20mm higher than those on road frames.