There is no queation in my mind: a day of casting my 12wt seriously wears on my shoulder/elbow, and I am not a milk-sop of an individual. Last year I moved up to my 10wt BVK to buck some stiff wind (really stiff). It was not pleasant.
The rod itself is not the issue....its the weight of the line you are pulling through the air. You get 40 feet of 12wt in the air and you have to pull alot harder to move it than 40 feet of 8wt. Yes, a good double haul helps. Lefty told me at last year's Fly Show in Lancaster, PA that he avoids anything above an 8, and I don't think his technique is a problem.