I have very heavy project (run good on pc, but slow on iPad2) and I think to upgrade to iPad3 to improve performance. How many difference in graphic performance there are between iPad2 and iPad3? Improve things?
This is a pretty darned broad question that I don’t think anyone will be able to give you a straight answer for. Everything doesn’t rely on the hardware alone, if anything, your code and what type of assets you use can play the biggest factor in mobile development. Not only that, but the powerVR chips used in iOS devices are tailored in a different way than other mobile platforms, so you have to take that into consideration as well.
My personal experience, is that (obviously) the iPad3 trumps the iPad2 pretty heavily, but it all comes down to a balancing game. If you’re running a game that is heavy on effects, these may have to be scaled back to get higher resolution models in, and so on, and so on.
Here are some links that may help you out, they’re not directly related to Unity but game development in general:
http://www.iphonehacks.com/2012/03/ipad-3-cpu-and-graphics-performance-compared.html
An interesting non-technical look at game comparison when developed for both iPad2 and iPad3
I know this is an old post but from my current project I can tell you that iPad 3 under certain scenarios will actually perform worse than iPad 2. Although iPad 3 is running at 2048x1536, it is using more or less the same processor to push 4x the number of pixels to the screen. In my current project, iPad2 and iPad Air runs smooth but iPad3 is pretty jittery and cannot maintain even 30 fps.