First off, it's better to overpitch than to underpitch. From what I know of yeast, which is not a ton, the only pitfalls I can think of would be a lack of yeast character in the beer. Many of the yeast flavors and esters are produced right away during reproduction so if you are overpitching their will be less reproduction which would mean less yeast character. This would be mostly a problem if you are using a yeast that contributes a lot to the finished product and less a problem f you are using something super clean and crisp. I think at least, would like to here someone else's opinion or at least validation of my theory.