We have several Dell R720 + 2 x Grid K1 where we are experiencing Post error messag when setting the Memory Mapped I/O above 4GB to Disabled. See attached screenshot.
I had initially posted this error on the Citrix XenServer GPU forum
Right now we are getting around this by setting under "miscellaneous settings" the "F1/F2 prompt on error" setting to be disabled (as suggested by Tobias).
All our 2 x K1 R720 configs are using the Sandy Bridge cpu (E5-2660). I do not have an easy way to test on Ivy Bridge config (E5-2680v2) as these systems are in prod with 1 x Grid K2 (GPU passthrough under XD 7.1).