no1 will be able too, there are no apps available that would tax or slow down the ipad2 or ipad3, a faster processor doesnt make any difference other than processor % used, once something runs as fast as it can, then it stops there, nothing improves just because the gfx/cpu is quicker.
same applies to your PC or Mac, if your old one ran the latest and greatest games at full speed at your monitors refresh rate which is probably 60hz for most flatscreens, then getting a pc/mac which is 3-4x faster in all aspects wont change your games speed in any way, it will just stay the same, the apps full performance will remain full performance, only the ipad4 will consume less resources/cpu to reach the apps full performance.
what you need for such a test is an app on the ipad2/3 which is noticably slow to the eye, then yeah if you find such a thing then the ipad4 will make a massive change, if not then theres no difference at all, same aplies to web browsing and other tasks, more cpu/gfx speed doesnt = faster internet.