Peeps were giving Big Mom benefit of the doubt against the likes of Shanks and Akainu before she started fighting on the panel. There are reasons to believe Mihawk'd have top-rated attacks even if he hasn't done anything. We have the other swordsmen—who're obviously weaker than him—to scale, at least vaguely, where he stands in the power spectrum. You don't have anyone like that to compare with Big Mom, and she remained unimpressive even after going all out. Mihawk, on the other hand, has, by far, the biggest raw power feat in the manga to date. Add a bit of CoA to it, and then pour in some CoC coating, we'll be redefining the power structure when he starts using named attacks.