Punk certainly hasn't become bigger than the belt but I will say as a heel CM Punk does not need the belt to be an effective heel as from 09-11 I always felt CM Punk was far and away the best heel the WWE had. When Punk had his SES he had as much heat as Jericho did when he knocked out Shawn Michaels wife, its very rare you can see people garner that much heat on a consistent basis. He's not a bad face but his character screams heel, its just so easy to hate a character like that.
All in all as a heel I wouldn't say that CM Punk needs to be champion to be relevant, he's good enough to be relevant on his own. Just give him a good face to work with and Punk will be fine.
I agree that Punk has been a good heel but when you put it that way I have to ask: who did heel Punk put over? If Punk is a legendary heel where is the PPV buyrates and tv-ratings to back up that claim? What did CM Punk accomplish as a heel?
Many people say JBL was an awful heel but he put Cena over. People say heel Hogan was past his prime and boring but he won the monday night wars for years.....