How do you enable HiDpi mode for unsupported monitors like the Dell P2815Q in Mavericks?
The answer is here: 

Basically, per Apple’s website you:


Open Mac App Store


Download XCode (free)
Follow their instructions to download Graphics Tools for Xcode package
Run Quartz Debug, following the instructions here on StackExchange, making sure to note that you will have to make each settings change and log out of your account each time, and if your computer has multiple accounts you may have to do the same on each one! (What a pain!)
Somehow it should automagically work out that the next time you log in, you can choose HiDPI settings in your Displays control panel.