I understand the limitations in that I can't run it at full resolution at 60hz, but can I run the monitor at 2560 x 1440 60hz? I'm trying to replace an older Apple Cinema Display and want to keep the resolution the same. And is my assumption correct that at 2560 x 1440, it'll just be more crisp like a retina display? Thanks!
I can't answer your first question with guaranteed accuracy - there is indeed a lot of confusion about that because it seems to depend a bit on the display model itself, the version of OS X, and whether you're open to possibly needing to use a utility like SwitchResX to mess around with settings.
What I can say is that my experience with the Dell P2715Q (4K display) is that it looks terrible when not using the HiDPI resolution settings.
What you need to understand is the difference when discussing "scaled" HiDPI resolution settings, so if you want to use 2560x1440 resolutioni on a 4K/HiDPI/retina screen, you want to use a
HiDPI setting of 2560x1440 which means you're still getting the full native 4K resolution (3840x2160), it's just scaling the desktop to the "equivalent" of 2560x1440... so it should be crisper compared to the TB display. Unfortunately, that's probably
not possible on the mini @ 4K
60Hz... you be stuck at 30Hz (though using SwitchResX, you
might be able to get that up to ~50Hz which some users find "good enough").