I'm using U2515H too and I found that, with 2560x1440, the UI is too small it hurts my eyes in the long run.
As mentioned by Sanpete before, use this tool here
https://comsysto.github.io/Display-...or-with-HiDPI-Support-For-Scaled-Resolutions/ to generate custom HiDPI resolutions. 1920x1080 worked for me best. Scroll down for full instruction (and read
https://comsysto.com/blog-post/force-hidpi-resolutions-for-dell-u2515h-monitor for more detailed information, including how to disable SIP)
And it works fine in Sierra. After doing all the steps, you can find your custom resolutions by pressing Option button while clicking on the 'Scaled' option on the Displays menu on System Preferences.
By default, the tool is already set up for U2515H monitor, but just double check because the DisplayProductID can be different depending on how you connect your Mac to the display (from HDMI or DisplayPort).
Just note, because it's not written in the instruction, if you want to make a custom HiDPI resolution, you have to make 'the doubled version' of the desired resolution. For example, if you wanna have 1920x1080 in HiDPI, you also have to make 3840x2160 non-HiDPI using the tool. If it's 1440x900 in HiDPI, then also make 2880x1800 in non-HiDPI, and so on.
By the way, just in case you don't know, HiDPI is just the technical term for Retina-quality (or you could say, 'Retina Display' is the marketing term for HiDPI display, although 'Retina Display' in Apple world involves more than pixels/ppi).
Let me know if you have any question.