I thought this might be good information for those who might be thinking about getting a new 5k iMac (Late 2015).
I just got mine (27") last month and today got PS CC 2015 installed and low and behold this thing does in fact have a 10 Bit (30 Bit total) Display. Very cool.
I discovered the setting both in Photoshop>Preferences>Performance>Advanced, enabled it, and then did the following...
In Adobe Photoshop CS6 create new file
900 px wide and 300 px tall
RGB color and 16 bit
Use gradient tool to create gray scale gradient from RGB: 64/64/64 to RGB: 96/96/96
Safe file as 16 bit tif file
I then turned on/off the 30 Bit setting in PS and sure enough, the image was banded with 30 Bit off and smooth with 30 Bit enabled.