If they had
superpowers, somehow I doubt they'd stop at street-level crime...
If an entire gang of supers went after the Joker, he'd probably hide (or turn himself in and start tormenting the police for the hell of it) causing instability in the freak circles until some mighty villian showed up to sit in the gap, and Lex would hire plus plus supervillans...so either in kind or number there's going to be escalation. Also it'd be pretty depressing for PCs to punch down as they levelled up unless they really really loved making mugger hummus.
I'll stop corbie-picking the concept, though, since you know the only way I'd ever be persuaded to play a supers game would be if everyone had to pick their PC idea from their first (preference-filtered, naturally) roll on this thing:
http://www.seventhsanctum.com/...ame=superheronameorg