HiDPI on external monitor (Dell u2515h) with Macbook Pro
I encountered the very same problem while setting HiDPI mode for my Macbook Pro with SwitchResX. As the HiDPI resolutions didn't fit, I made my own ones.

Here's how it goes:


Follow the instructions on SwitchResX's website on how to temporarily disable "System Integrity Protection"
On the SwitchResX panel for the DELL screen, go to the "Custom Resolutions" tab and create your desired resolutions under the "Scaled resolution" category.
The pixels x lines you scale to are 2 times the "feeling" you want to get, e.g. 3840 x 2160 for a Full HD (1920 x 1080) feeling.


Disabling the SIP made the 720p resolution you mentioned work for me as well.