So I got impatient and decided to buy the Dell UP3216Q... ;)
So far so good! The sharpness is amazing.... lots of clarity compared to the U3011 for images zoomed out. It's a bit more like looking at images on a retina iPad.
I am in Win 8.1 so scaling is basic, but with a small scaling boost everything is basically fine. Max UI is fine, no issues at all, although at the optimim scaling level it's too blurry to use, so max 2017 will fix this (so I'm told).
The monitor itself is very nice. Light and looks good, but the bezel is big, for sure.
Colours and accuracy etc. all seem fine. Compared to other monitors in the office it's fine.
One drawback (kind of) of 4k is that your usual test renders at 1600/2000 are too small on the screen to see properly, so you end up needing to zoom into them. They still look ok when you do that, but it's not actually ideal. I guess the solution will be to work at higher resolution for tests, something more like 2k>2.5k, which will add to rendertimes, and is not ideal. It's an interesting connundrum, actually...
The Dell calibration system with my i1 Pro was a NIGHTMARE and crashed all the time, BSOD etc. Unusable. I think it might be related to my GTX970 somehow. I ended up using a Spyder Pro 4, which worked fine. The calibrated result was exactly the same as the factory SRGB preset! So I'm pretty confident it's all good to go right out of the box, which is very nice.
Price is really, really attractive too. £750+VAT. Bargain, I think!
I'll post more thoughts later. I can't compare it against any other 4k monitors, but so far, so good :)