Twitch's passive

So got a couple games in with Twitch recently, and it struck me just how bad his passive is. Now i will admit i'm not great with him, but how is a passive that does 2-6 true damage per second per stack, maxing at 36 true damage a second, even remotely good enough? With just base healing enemies negate most of 's passive. Even at max stacks going for the full time (6 seconds), you are doing a resounding 216 true damage. 216 damage at level 16 or higher is by no means impressive. That is probably about 10-15% of the squishiest champions health pool BEFORE you factor in their natural health regeneration, and against more health rich champions 216 damage over 6 seconds is barely a drop in the bucket.

Would it be possible to buff and/or change this champions passive? Maybe make it so that it totally shuts off health gain while active or turns healing into damage while the effect is active. Imagine picking Twitch into someone like Soraka or Mundo, where if you land your passive their insane healing turns into a huge draw back instead of a boon for them.