Ha, evidence? I've got none of that, I'll give you that.
I can see I'm going to need even more qualifiers to wiggle my way out of saying they'd sell a shload of tickets.
First off, I wouldn't pay festival prices to see Faith No More, so I'm not sure why I'm even trying to make this argument for them. The only band I'm that big of a homer for is Hum, who fit the exact scenario you mentioned.. fans who think they're band is big enough that everyone cares, but everyone really doesn't. I at least realize Hum would probably only sell about 3 to 4 thousand tickets if that, to a huge festival.
Faith No More certainly wouldn't be a headliner and won't draw that level of people. They'd draw on the same level I believe Iggy and the Stooges did last year (coincidentally, the only reason I went to V-Fest last year). But yeah, I'm fairly certain that as a mid-level band, playing a few slots before the headliners, they'd sell a good deal more tickets than the percentages indicate a band in that slot should.
Okay, a shitload of tickets.. maybe an exaggeration when you're thinking in the context of the entire festival. But I firmly believe they'd sell their fare share and then some.
Fair enough?