That would require the application to render in the lower resolution(otherwise I fail to see how it would require less CPU/GPU power). Technically it's possible(for example OpenTTD can run in lower resolutions IIRC), and it might use less battery. However, the application still *needs* to do all the calculations but either downscale it(as you proposed) or simply render it in a lower resolution. Or you can make the application less graphical intensive since you can't might not see everything if you lower the resolution.

My 0.02£.