MacBook Pro 2015 15" with ASUS PB279Q 4K @ 60Hz I'm looking to see if anyone has been successful in running the ASUS PB279Q at 4k 60Hz via mini DisplayPort on a MacBook Pro 2015 15".

Running via HDMI works at 4k with 30Hz only, likewise setting DisplayPort to 1.1 does the same. As soon as you set the monitor to DisplayPort 1.2 I get a no signal on the monitor with the MacBook Pro screen flashing like it can't make up its mind.

I'm running up to date OSX Yosemite have tried a genuine apple thunderbolt to thunderbolt cable (mini dp > mini dp) as well as a cheaper mini dp to dp. According to https://support.apple.com/en-au/HT202856 4k should work as expected using SST.

Any further ideas?
In order for your connected display to use DisplayPort 1.2 you also need a compatible cable. I'm not sure about the third-party cable you were using but I believe the Apple Thunderbolt cable only carries DisplayPort 1.1a signals. 

If you buy a third-party cable that's confirmed to support DisplayPort 1.2, 60Hz MST should work properly. The DisplayPort products database may help in that regard.