Since the pixels are multiplied by 4, and the GPU only by 2, does this means the real world performance (in games) of the iPad 3 GPU should be half the performance of iPad 2?
That sort of depends on where the bottlenecks are, and what multiplying the GPU by 2 means. Basically, there's not enough info to say, and it'll probably require some benchmarking to tell.
I doubt they are using the same GPU that's in the iPad 2,I suspect that they went with a higher end GPU and made it Quad. We will have to wait for a teardown to know with 100% certainty though.
A teardown won't tell you anything: it'll tell you that the GPU is in a chip called the A5X. You might be able to get a little more by pulling out an electron microscope, but iFixit doesn't go that far, yet. :)
It'll be the people doing software sleuthing that'll get you the answer.
However we are fairly confident that Apple didn't build their own GPU -- they would have bragged about it if they did. There are a limited number of GPUs IP blocks available for licensing, so that does mean that speculation can be relatively accurate. Their use of the term "quad-core" makes it highly likely they're using the SGX543MP4, twice what is in the iPad 2 (although perhaps higher clocked or with better memory bandwidth). That's also the GPU in the playstation vita.
