I ordered a new Mac Pro but not early enough for it to arrive in a timely fashion. The three Dell UltraSharp 32 UltraHD displays I ordered to go along with it, however, arrived almost immediately. So, I did what any self-respecting geek would do — I connected them to my Retina MacBook Pro. Here's how I got it to work!
I tried plugging all three displays into the two Thunderbolt ports and the HDMI port, but I can't get more than two displays running at 4K at one time. I can plug two 4K displays into either both the Thunderbolt ports, or one of the Thunderbolt ports and the HDMI port, but if I plug in all three at the same time, the HDMI display goes blank. So, I've settled on two 4K displays on the Thunderbolt ports for now (still amazing that the MacBook Pro can push all those pixels!)
You get used to the 30Hz refresh rate pretty quickly. The cursor lag is annoying at first, but eventually becomes the new normal. Nonetheless, I am looking forward to the MacPro and, eventually, proper MST support to enable 60Hz.
One issue with 4k @ 30Hz is that Chrome for some reason becomes unbearably slow. Pages takes minutes to render. Disabling BeamSync fixes that, but causes some tearing while moving windows. An acceptable trade-off for now.
Resolution wise, 3840 x 2160 even at 32" yields a tiny interface (137 dpi vs 108 dpi on iMac), but surprisingly comparable dpi to the 11" MacBook Air (135 dpi). The difference, of course, is that I often sit really close to the MacBook, and more than arms length away from my desktop monitors.
Most apps support scaling well when it comes to text, but interface elements remain at their native resolution. Again, it's surprising how quickly the tiny interface becomes the new "normal".
By installing SwitchResX you can enable 1920 x 1080 HiDPI mode but everything becomes comically large at around 69" dpi.
Using custom resolutions I tried to enable 5760 x 3240 HiDPI mode, shrunken to 3840x2160, which would yield an effective 2880 x 1620 (103 dpi) desktop, but was unsuccessful. Ideally Apple will eventually natively supported interpolated resolutions, like they do on and the Retina MacBooks.
Sitting arms length away at 3840 x 2160 at 32-inches, I can see no pixels. The screen looks identical to the beautiful Retina screen on my MacBook Pro. On my old 2560 x 1600 30-inch setup I could definitely see the pixels. So, overall, it's a huge improvement.
If anyone has any questions about the setup let me know., and anyone else is running 4K off a MacBook Pro, or a Mac Pro, tell me — how's it working for you?