It would be awesome. I think Penn would have the most trouble, because he is the most naturally suited to his current weight class. However, if he were to dedicate himself fully to it, and put on some legit weight (not flab), he would be a serious threat to pretty much everyone at WW.
GSP would have to put on more weight to maintain his current fighting style/gameplan. He like to dominate people through superior strength, conditioning, and technique. If he didn't add some muscle, he would get overpowered buy the heavier guys. His wresltling technique would still be there, and he would therefore still be a force.
Silva has already shown that he can dominate at LHW. He would have the easiest time because he already fights far below his natural weight. His striking would still be spades above all of his opponents except maybe Machida and Shogun, and his sub game would still be good. Therefore, he would dominate all LHW's except maybe the top couple fighters, who would give him an even challenge IMO.