I don't play a grav controller, but I can answer the question you had about Tornado. Basically, you know it'll cause havoc against roaming mobs by throwing and disorienting them, but against crowd controlled ones, it becomes a major damage-dealer.
A tight-packed group of minions sitting under a freezing rain will probably be killed just by the DoT of Tornado, assuming they have some gravity control that raises their magnitude to be knocked up. As a storm defender, I know that I was able to slaughter purple Rularuu portals back when they were "invading" using Tornado, because those portals would get hit over and over again by the damage component.