Does this increase battery life? Less pixels to compute?

[nodingneu]

No description provided.

[rednoah]

You could try running a few tests yourself. I kinda doubt that it makes a noticeable difference though (for anything other than gaming).

[Argon-]

This can indeed be the case. When you choose a "looks like" resolution that is exactly the native resolution of your display divided by (currently 2 on all Macs), one scaling step can be skipped.
However, you don't need RDM for this. You can select such a resolution from system preferences.