Initial thoughts...

[Cynaidh]

I wonder how it would fare in a build that maximized Int, Pres, Chi? If I find a suitable glad, I'll give that monstrosity a try

Thats what Drayn is, 100 int, 92 pres, 91 chi - after stat trains he is almost fully trained now only a couple of points in str left

nearly doubles their endurance usage per attack.

Yup i have seen this also opponents spending 10-12 endurance for an attack rather then the normal amount, it doesn't seem to happen every attack though, they still get attacks at the normal cost too. I am wondering if it has something to do with their current endurance level it seems the higher drains 11 and 12 happen at the start of a fight when they are full, and towards the end it drops to 3-4 per attack. That was vs a Warrior, the next fight vs a speed rage there was no effect at all it seems on his endurance cost, every attack the speed rage made cost him 2-4 endurance.... the rage made 7 attacks and was 72/94 (he was also hit twice by drayn). And 7 attacks with 2 rapiers seems like a normal amount in fact he was on the ground after the first attack, so 1 normal attack, then 6 attacks from the ground (drayn was not stunned either).