I prefer the idea of a feet slot in addition to legs slot than the Dex stat; it seems less complicated and more interesting. I'd suggest adding some other jump types as well (some have already popped up in
http://www.arcengames.com/mantisbt/view.php?id=7870) to the slot, so there's a variety in jump choices.
I'd agree that Stone Legs should remain as a jump - negation enchant (so it goes to the Feet slot - rename it to Stone things on your feet, maybe?)
Legs could do with some new enchants if this happens, though; off the top of my head, Firing Position (increase damage dealt while crouching and makes it so when you fire when crouched, you stay crouched instead of standing up to fire and then crouching again), Ground Anchors (reduces knockback you suffer), and Swimmer Fins (no reduced mobility when in liquid) would be good additions, since they're both roughly the same power level yet still useful.
If dex is retained, I wouldn't want it to limit horizontal movement when jumping, but the fall damage and detectability parts would be decent.
More mana for Niche Air as a feet slot enchant sounds okay, though I'm inclined to suggest that it should increase (upwards) jump speed as well; that way, its more useful in a jump form as well since it allows you to get out of the way of incoming projectiles better.