The 675MX is a 75W TDP GPU. Not in a million years. Doesn't matter how cheap it is, it is only found in big fat Gaming Notebooks.
For a significant speed bumb you'd need to wait for a process shrink with the next in 2014. The only thing a speed bump would get is a 760m or 8800M basically the whole mainstream notebook GPU rebrands. Other than slightly different clocks and newer drivers there won't be any difference.
From a speed perspective the next update will be pretty boring as far as I can tell.
An Intel HD 4600 is also not double the 4000. It refers probably to the 20 EU Version which will only be somewhat faster but the bigger one with eDRAM will be the game changer but it will only be offered in dual core chips (my guess). Quad Cores get dedicated GPUs they don't need to waste die space in the equivalence of 2 full cores + Cache for a iGPU.
Little mockup from what I would expect the Haswell DIEs to look like given all the Info from IDF (see Anandtech) and the rumors of 20/40 EU versions aka GT1/2 or sometimes 2/3.
This is the Ivy Bridge current DIE.
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This as I guess the Quad Core DIE looks like with the HD 4600 (20EUs).
So very little difference and that would be the one that would go into a 15" rMBP.
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This would be the Dual Core with the best iGPU. This would be the 13" rMBP chip and quite an improvement. If the 40EU GPU is true that is one massive change in respect to GPU vs Core space. Even more GPU than the AMD Fusion Chips. Now if Intel just would provide decent drivers.
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Given that the long Quad Core chip is quite long and still has to fit on the package I think even trying to make the big GPU fit on the Quad would be a problem. They would have to reorganize the entire layout which is probably not as easy as it seems as the ring bus would then have to go around the Cache if the GPU was below the cores.