I have a 13" MBP (Early 2011) plugged it to a Dell U2711 monitor via DisplayPort.
I have successfully enabled HiDPI mode, using Quartz Debug the problem is that the screen won't run at the HiDPI equivalent of the screen's native resolution. i.e 1440x720 HiDPI (2560x1440). The screen's maximum resolution of 2560x1440 is recognised & working.
The best it will do is 960x720HiDPI (1920x1440), which seems strange as most of the documentation suggests HiDPI will only work at pixel doubling resolutions.
Is it possible to force the MBP to run the display at 1440x720 HiDPI on a screen that supports 2560x1440 native pixel resolution?