I have a Mid-2014 15" retina MBP with an NVIDIA 750M running 10.12.5. I am outputting to a Dell P2415Q via an mDP to DP cable.
I have currently attempted a few solutions. I've played around with SwitchResX. I've run this command:
sudo defaults write /Library/Preferences/com.apple.windowserver.plist DisplayResolutionEnabled -bool true
And I've even used the EDID patcher that others have used to force RGB mode on their displays. Option-Clicking the Scaled button gives me resolutions with (low resolution) next to them. I would like to go 1440x2560, but the (low resolution) option looks nowhere near as sharp as the retina scaled 2560x1440 I can set the display to in landscape orientation. If anyone knows how to obtain these scaling options while rotated I would appreciate it.