I didn't see anywhere mentioning if the 2013 Mac Pro supported *2* 5K Dell UP2715K monitors. Apple's site only mentions the ability to support the model. I have 2 of them, so I figured I would try. I had the first one already working as my main display. When I first connected the second, it showed up as detected by the Mac Pro, but the screen was black. After disconnecting one cable, and getting it to come up at 2560x1440, I plugged the other cable back in, and it came up at full res. So, while only supporting 2x 4K monitors at 60Hz, the Mac Pro will also drive 2x 5K at 60Hz. That is nearly double the pixels. I have not had a chance to test much with regard to performance or anything. As a side note, I verified that it does work as a second monitor on a 5K iMac with the m295x, but it makes it reeeaaaallly slooooww for most things. It was also pretty flaky, blinking in and out. Now, if Apple would just fix the detection at boot, so you can get the boot logo and such, it would be perfect. If you hold "option" at boot for a very long time, you do get a screen, which appears to be at 2560x1440, or something similar. When I get back from the weekend, I'll test some more. I am not likely to keep this setup, as it eats 2 extra Thunderbolt ports, and I only really have 1 spare.