I like 2560x1440 (not scaled, not HiDPI) for gaming, just for the faster frame rates even though my graphics card can do 4K fine.
First, make sure HiDPI modes is enabled.
http://apple.stackexchange.com/ques...de-on-external-display-in-os-x-10-9-mavericks
You should see a 2560x1440 HiDPI mode now. If not then you can make HiDPI resolutions with SwitchResX by creating a scaled resolution that is twice the height and width of the desired HiDPI resolution. So for 1440p HiDPI, you need to create a scaled resolution of 5120x2880.
I like 5696x3204, because it gives me a HiDPI mode of 2848x1602 which is similar to the height of my old Apple 30" Cinema Display (2560x1600).
In SwitchResX, double click the HiDPI resolution in the Current Resolutions list to make sure it is using the 3840x2160 as the active resolution. If it isn't then set the "Scale resolution base" in SwitchResX to 3840 x 2160.
I'm avoiding 5K for now.