They are weighted to the exact same(overall weight is lighter by 5g compared to my PS 6.1 BLX, balance - both HL 10/11, SW - I don't know but mostly feels somewhat the same. I do notice a difference ONLY on put away shots where the ultra is slower by maybe 1/3 of a sec, and my shots are out by a small amount). I agree with your assessment that I was trying to move it as fast, and I agree that its most likley due to the headsize, and beam width. I'll rest for a few days and try it again, and if it starts hurting again, maybe its not for me..
Edit : Thinking about it more, I was trying to hit with more topspin than usual, but not by an extreme amount, due to the difference in string pattern, but the muscle that hurts doesn't seem to be a contributing factor when I move my arm in the way I generate spin, but who knows. So many interconnecting muscles, so that may have contributed to it?