I presume that model screen must use MST if you are at 60hz? To get that working, did you have to manually install the Intel video driver?
I have the Dell p2715q running at 50hz with that setup. Works perfectly fine - and with the docking station you have the potential to run two of these displays - although at this point I am enjoying the move from two 1920x1200 to a single 4k display plus the Surface. Lots more desk space!
Exactly.
It is a mini display port (docking station SP3) -> display port (Dell) connection.
I use the latest Intel driver for that, but Microsoft march update finally enables 60Hz MST 4K also on "their" version of display driver as well.
I tried both: you can't go wrong either way.
Only minor annoyances so far:
1. I had to disable automatic refresh rate reduction on battery to 48 Hz in the Intel driver's properties. Because sometimes Dell couldn't recognize the signal in MST mode and was just flickering.
It happens, because Surface sometimes drops the refresh rate to 48 Hz, since it not always being charged in the docking station. As I said, it happened sporadically and always, when Surface woke up from sleep.
It is not a big deal, because this 48Hz reduction doesn't save you any significant amount of power. Display brightness does much more.
2. Sometimes driver seems to "forget" the 4K resolution of the extended desktops on Dell, again after display sleep or just being off. So you have to re-enable it or switch the monitor off and on again. I'm still not sure, what exactly causes this - graphics driver or missing monitor signal.