I would expect target display mode to be there, but to require Thunderbolt as input (like the 2011 iMac's).
I'd be less certain that you'll be able to input retinal resolutions -- especially if the 27" has a full 5120x2880. Assuming I did my math right, that would require ~20GB/sec, and the Thunderbolt only has 10GB/sec for the video stream...? Even 21" at 3840x2160 looks like it wouldn't work -- ~12GB/sec.