It depends on which AIO, and orientation of the radiator. It does seem a little tight, but worked ok for me. I used 2x Corsair H75.With dual AIOs in the M1, it seems like the hoses are an issue, blocking up the airflow...
In the Shift, the extra height helps alleviate the issue with hose lengths...?
In the Shift, the extra height helps spread out the length of the AIOs but the bottom front radiator seem to breath better than the bottom radiator.