From what I've heard, it's hard to over pitch. You'd need to pitch a ton of yeast, like double or triple?. It would ferment quicker with more cells to do the work. Many of the yeast derived flavors (good ones) are created during the growth phase, when cells are multiplying. If you overpitch, you shorten the growth phase and reduce those flavors. This is probably more important in styles with dominant yeast flavors. There might be other effect too.