Question is though how the 750m compares to 680MX. The 750m has higher GPU core speeds but the 680MX has larger bandwidth and higher memory speeds.
I don't think the 750m will replace the 680MX but Apple might update the low-end from 660m to the 750m which will make the low-end 27" a lot more appealing.
Although, knowing Apple, they probably won't do that anyway unless they can update the high-end as well since if they update the low-end, it will close the gap between the low-end and the high-end, making people feel less "need" to pick the high-end.