I'm currently playing a tornado caster, I started the game in a version where Tornado was at its most nerfed (the new cooldown/radius/etc mechanics and 6 damage dice) and I later upgraded to the latest trunk version.
The nerfed Tornado seems pretty good for the typical 3-rune stuff. It seems to destroy most mid-late game monsters, but on its own it's not enough to kill dangerous uniques or high HP monsters like shadow dragons, titans or orbs of fire before they have time to do significant damage.
On the other hand I tried to fight Tiamat's band and had to give up twice: after 2 Tornados and while meleeing her the whole time (with a character that could take anything but the highest-end monsters in melee) she wasn't even at half HP and some of the tougher draconians accompanying her were still alive (and I was nearly dead
). Considering that Tiamat is on par, at least HP-wise, with the weaker Hell/Pan lords, I doubt I could have relied on Tornado to do anything significant in post-endgame rune floors, let alone in Zigs.
The new mechanics, especially cooldown, only seem justified in light of being able to cast it in time for branch ends. I doubt this low damage version of Tornado but wihtout cooldown would be overpowered in Zot:5.
I then upgraded the game to the latest trunk, and my impression is that Tornado is back to 0.8 levels of craziness. Here, the new restrictions are almost meaningless: cooldown doesn't matter when almost everything dies with one cast, altough I guess the expanding radius might matter a lot on Tornado's effectiveness in Zigs. Speaking of Zigs, I'm looking forward to try Tornado in them, I have a little issue to settle with some mummies...
Overall, I think the best choices would be either the low damage version but with no cooldown (like it was after the first 0.9 nerf), or a middle ground damage nerf while retaining the cooldown. I have no idea how it could be tweaked so it isn't overpowered in a 3 rune mid-late game while still being significantly useful for the hardest extended endgame stuff.