They have the same exact CPU with the same amount of instructions per clock so there is a firm limit there... who cares if the GPU can accept more instructions if the system is CPU bound? If Xbox ONE is more efficient and can keep its 12CU fully utilized but the PS4 cant keep its 18 fully utilized because of the CPU, then you paid for "wasted" silicon unless you can effectively implement GPGPU.
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If they are CPU bound why would Sony spend money on GPU resources that can't be accessed? I agree they have the same CPU but they don't have the same bandwidth and I think that is key to understanding the engineering decisions regarding the GPU.