The iPad 3 in particular has the retina display, but it doesn't have the souped up GPU of the iPad 4, and this can be an issue.
Yeah, 2048x1536 is a lot of pixels. I mean, the iOS emulator doesn't fit in my monitor at this resolution so I could see how this would be a problem for something computationally intensive. In my case, I'm working on a tile-based 2D game in which there will never be more than, say, a dozen small sprites in motion at once. If 3rd generation hardware can't handle that then something is really wrong.
My biggest problem right now re:redoing all my art for 2048x1536 is the following: