By 30% smaller, I am assuming you mean AMD's 20nm process opposed to NVIDIA's 28nm process. This would suggest that AMD would be able to fit twice as many transistors into the same area as NVIDIA, and therefore should in theory be twice as fast, for the same cost. Or, just as fast for half the cost. Or, just as fast, just as expensive, but using 2/3 of the power. In practice, it won't work like that, but it should still be a fair improvement.
Either way, there is no reason Apple cannot include a GPU significantly more powerful than even the very expensive Iris Pro, whether it's a NVIDIA 750M/760M, or new Radeon 9770M.
That said, we'll just have to wait to see if/when/what new mobile GPUs get announced.