Right about the Recharge with Storm.
I don't think Plant/Storm should beat either Ill/Storm or Grav/Storm in theory, but in practice different players may be better or worse at DPS management, or one may have a better build, and Plant isn't so far behind Illusion or Storm (or Fire) that it can't put up DPS that's close enough to them. I would expect Ill, Grav, Fire, and maybe Dark to beat out Plant with the same player and level of build, but a good player and a good build can make a difference when it's not the same player using the same kind of build for both characters.
I always heard plant and fire referenced as the most damaging sets at least the old forums and the splattrollers/farmer threads? Are you specifically talking about damage vs single tough targets like AVs? I have heard about Gravs control but not its damage I guess. My guess is creepers may have a couple loopholes that allow it to scale in ways it really should not supposed to? It has a lot of options for socketing procs, - res, hold, and a lot of damage slots. If it works like masterminds than that single enhancement gets its own ppm for each creeper tentacle?