On a 4K display like he mentioned, "Looks like 1080P" HiDPI mode would not require scaling. OSX would output the full 3840x2160, but because it's HiDPI mode you would get the effective "real estate" of a 1080P display.
The only thing that would be scaled would be applications which aren't HiDPI-aware.
Got it. Looks like I'm waiting for 4K