20% while hexed is I believe what the op meant to say? Yes it is possible and it is max. But it is crappy. Too situational.
I had a very nice gold bow that dropped the other day with req8 and that mod and I just sold it to merchant.
I don't recall ever seeing a "vs" anything as an inherent mod. There is While in Stance, While Enchanted, While Hexed, [Always], While Health is Above 50 and While Health is below 50.
Notice that these all say "While". The ones with the poor situations max at 20%, the others max at 15%. This is all on dmg of course, not energy.
I would love to see screen shots or see them in game. I am not saying I can't be wrong here, but to the best of my recollection, I haven't seen a vs. hexed inherent mod before. That would be a nice weapon as you can control it more.