I think your question was directed at me as I am having the problems ;-)
Dell at full 3440x1440 and SP3 at 2160 x 1440, connected with the Mini DP cable that came with the Monitor. 100% and 150% both (Recommended)
Same scaling problem I see when connecting it to either the Surface Dock or into the Mini DP on the side of SP3. Blurry MS office apps on one monitor and in Chrome. Also I do see the Dell U3415-W occasionally go blank momentarily when connected via the Surface Dock. When this happens it does not blank ling enough for the Dell to pop up its "going to sleep mode" message or whatever it is. Just blank for maybe 2 sec then back on.
It is promising to me that you guys are assuring me this hardware combination can work. Thanks for this. Gives me hope I haven't just wasted a heap of money.
So I am now thinking drivers. I have previously been running a custom SP3 resolution to mitigate scaling on twin Full Def screens. To do this I had installed Intel Display drivers . In device manage I see Display Adapter > Intel (R) HD Graphics Family ver 188.8.131.5231 although I did have the newer 4424 running yesterday with no difference.
What are Display driver versions are your systems showing in device manage please.
SP4 i5 I'm on now drives the U3415-W fine on MS drivers. Driver version is 184.108.40.20609 dated 3/20/2016