Power of 2 sizes can almost be forgotten... they are only relevant with tiled images (so not Sprite images), and in the near future mobile devices will also support non-power-of-two textures, meaning power-of-two is finally irrelevant and makes no difference at all.
In the mean time, there's not really any performance difference, it just affects whether tiled images can tile with best quality, which currently can only be done with power-of-two images.