Oakape, I have done the high altitude upset exercises several times in the 737 800 sim and not found any issue regarding flight control effectiveness. The force needed on the elevator will be large if trimmed nose up (for the reducing IAS prior to stall), but that is no different from at any altitude, but even without trimming, I didn't find any problem. I'm not a sportsman and never go to the gym, though I'm not a couch potato either - I just live an active lifestyle. So, a 5' waif may have trouble, but the average male pilot shouldn't have any issues. I can't speak for other types, though.