So it seems my late 2013 15" rMBP should be able to drive my P2715Q at 4K at 60Hz, but no matter what setting I fiddle with, I can only get it to be 30Hz. I use my Mac in clamshell mode. What am I missing? I'm using the supplied Dell mDP to DP cable and MST is set to Secondary in the monitor's settings menu. Thank you for your help.