And no word on support for GPUs at all. If you look at the product site, it only talks about capture cards, RAID cards and stuff. People think external GPU is simple as getting that thing and then slamming any GPU in it and then it would somehow magically boost your graphics performance. It's far from that.
First off, it needs extensive software support. Your internal display is still connected to the internal GPU(s) (if you have 15/17", then it is connected to the iGPU and dGPU). You will probably need something similar to CrossFire to push the load from the internal GPU to the external. Since Sonnet does not mention anything about this, they probably are not developing it for this purpose. Connecting an external display to the ext GPU might be easier from a software standpoint as USB GPUs have existed for quite awhile (even for Macs). That would eliminate the need for getting the int and ext GPU play along.
Secondly, Macs don't use standard PC GPUs. Macs use EFI which means the GPUs have special firmware in it. This would force you to use the GPUs that Apple offers, unless flashing works for the external GPU as well.
Thirdly, Thunderbolt offers only 10Gb/s while PCIe 2.0 x16 offers up to 64Gb/s. 10Gb/s should be sufficient for some GPUs but higher-end GPUs would most likely take a hit from the limited bandwidth. That might make this whole thing obsolete as 6750M could ens up being faster.
All in all, it's nowhere near as simple as some people make it sound like. That Sonnet box is a good start but you will need more than that.