I know you can 1280x720 HiDPI mode on the 2560x1440 27" iMac display simply by enabling HiDPI modebut what I want to do is is get a higher-quality 1920x1080 scaling enabled. I haven't seen or heard of anyone being able to do this, but I'm sure there must be a way to accomplish it. For example, I do know that when I hook up my MBPr and choose display mirroring, the display ends up showing a HiDPI version of of a 1440x900 or 1680x1050 or 1920x1200 display. So clearly the 27" iMac (and in fact any 1440p display) is capable of handling Retina-like scaling. Now, I know this wouldn't be true retina, because you'd still be closer to the display. But for my purposes, it would be the ideal resolution. I'm sure there is a worthwhile debate about whether or not this is desirable, but I'm not interested in debating thatwhat I'm interested in knowing is if anyone has either accomplished this or has any ideas of things to try. Obviously I'll report back if I figure it out.