As I said in my earlier post on 6th February:
"I have got my Mac Mini to display the full 3440x1440 resolution on the U3415W by connecting the mDP end to the Thunderbolt port and the DP end to the DP in port. I did however need to turn DP 1.2 off in the onscreen settings. I have a PC connected using the mDP port of the monitor and can switch between them using the onscreen menu." The miniDP port and Thunderbolt port connections are directly compatible.
Unless someone has already done this and posts to say so then you will be taking a risk. It seems likely that this will work as it has for me but Dell have specifically stated that the monitor has no support for Macs.