Right after I got the GM promotion, I did get the double attack as stated (attack went from +16 to +26, so the bonus from skill went from +10 to +20). However, the Damage stats stayed the same: 21-30

I tried extensive tests both with skill 10 at Master, and skill 10 at Grand Master. In both cases, I almost always never got higher than 25 damage. The highest I got was 26. The only difference I noticed at all, was that on Master, I got alot more low-scoring hits, ie 12, 14 damage etc... where as on GM they were almost all 25 like clockwork. This is probably due to the higher attack skill, though. I also did not once notice anything that resembled a 'Stun' effect. What a waste. My gladiator with 28-44 damage spear was getting 40's and 42's all the time. If the damage was indeed double 21-30, I should have been getting at least 50's and 60's.