According to Apple's KBase article on "Using 4K displays", the Sharp PN-K321 monitor is supported at 60Hz in MST mode if you're connected via mini DisplayPort to DisplayPort.
I've ordered a StarTech mDP-DP cable as Amazon's comments indicate that it works well (or well enough), just wondering if anyone has any experience with this or similar products.
Namely, do you have problems waking the Mac Pro from sleep? Does the display properly wake up? Do you notice any flickering or lag using Photoshop or any other 2D or 3D applications?
The Pro is already updated to OS X 10.9.3; haven't checked yet whether the Sharp display is on the latest firmware (v11301118).