You can check your resolution and HiDPI status by running this:

   system_profiler SPDisplaysDataType  | grep -C1 Resolution

For example, with my 4k monitor running at 3008x1692 with HiDPI ("retina") enabled, I get this:

   DELL P2715Q:
      Resolution: 6016 x 3384
      UI Looks like: 3008 x 1692 @ 60 Hz

Even with those settings my 16" MBP's idle GPU power usage is only ~8-9W. It makes no sense that enabling HiDPI on a 1440p monitor results in better performance, so this definitely sounds like a bug.

I recommend displayplacer [0] for changing and automating macOS display settings. It lets you set all sorts of configs not available in the GUI. Should be easy to have displayplacer enable HiDPI on a 1440p monitor. And it doesn't make persistent system changes or require disabling SIP (i think?) like that script referenced in the article.

[0] https://github.com/jakehilborn/displayplacer