Power usage differences are only applicable when you are talking about native resolutions, i.e all things being equal, a 1080 screen will use less power than a QHD one. But if you take a QHD screen and down scale it to 1080, power savings is marginal.. if any.
When these tools first popped up, I played around with them with my Nexus 6 and graphics performance actually suffered a bit because the GPU had to waste cycles converting. 720p is a more natural resolution for a QHD screen as it can pair sets of four pixels (like 4K to full HD)... Even then, power usage was more or less the same.
And 720 on larger screen... They are other far better solutions to save a little power than that.