Late-2013 15" MBPs support HiDPI with 4K Displays.
So it looks like you can hack 13-inch Displays to work with 1080p HiDPI at 52hz (http://forums.macrumors.com/showthread.php?t=1741440). Since it maxes out at 52hz, I'm guessing the problem is hardware-related (either rendering ability or bandwidth).
However, this means that you are just running at 1080p effective resolution. It seems like 1440p HiDPI mode is what we really want, and I don't believe this is possible (I couldn't get it to work, and the information I found on the web said the same). Hopefully the next-gen 13" MBP (Broadwell) will have DP 1.3 and more GPU rendering power, which will enable 5K displays.
So it looks like you can hack 13-inch Displays to work with 1080p HiDPI at 52hz (http://forums.macrumors.com/showthread.php?t=1741440). Since it maxes out at 52hz, I'm guessing the problem is hardware-related (either rendering ability or bandwidth).
However, this means that you are just running at 1080p effective resolution. It seems like 1440p HiDPI mode is what we really want, and I don't believe this is possible (I couldn't get it to work, and the information I found on the web said the same). Hopefully the next-gen 13" MBP (Broadwell) will have DP 1.3 and more GPU rendering power, which will enable 5K displays.