Yeah plus it kills all open apps when switching resolution, so I realise it's a pipe dream for now.
Like you say, would be a VERY cool geeky-but-actually-useful feature if Samsung could figure it out.
The biggest issue with changing screen resolution is how ineffective it actually is at giving you a battery boost.
Take your laptop, does lowering it's native resolution give you a noticeable boost ? Not really.
Important part .....
You see even if the content on display is running at 720p or 1080p, the actual screen itself is still illuminating all 2k pixels. Those pixels are not magically turned off saving you power, you are just seeing some resolution internal scaling instead.
Therefore if you are saving no battery illuminating the screen in lower than native resolution, where could you get any saving?
The answer is on CPU and GPU power, but this will only likely be any use if you are gaming and would likely manifest itself moreover in better frame-rates for games than actual improved battery life.
2D content that is not taxing combined GPU/CPU which is 95% + stuff we do (Twitter, Facebook, Dialer, Calendar don't really require too much GPU) on our phones will not provide you with any inherent boost by running them at a lower resolution as they are not taxing the system as is.
So really outside of gaming, reducing the handsets resolution will provide very little improvement overall and the 'image softness' that is inevitable when running outside of native resolution may be a bigger negative than any possible performance improvement.
So currently there is very little point running at HD/FHD on your QHD device as the gains will be so negligible battery wise, and countered by the negatives of loss of image clarity.
Just remember, You are still illuminating QHD amount of pixels regardless of screen resolution and hold onto that thought, consider what you mostly do with your device, and you quickly realise just how inefficient a solution to improving battery life that this scaling actually is ...
You would get FAR MORE battery improvement if that's what you are seeking by merely dropping your screen brightness down a few notches than you ever will achieve by scaling the internal resolution.