And as any viewport, it means the game has to render everything in the windows of that camera that is later shown in that viewport. The low res of the viewport reduces the GPU load on the final parts of the whole process, but doesn't reduce the CPU load that has to be done first. To lower the load on the CPU that viewport has to sacrifice some things, like SCS does with shadows for example. If not, the cost of rendering that viewport is almost the same of render the main point of view, with the main difference to be only the rasterization and beyond parts of the whole render process; as there's way less resolution as you correctly said.
So, again, in a game that bottlenecks almost any CPU on the market (unless you use a joke of a GPU chat could be bottlenecked way before) because its limitation on the mostly-mono-core architecture; it's not a good idea to do such a change that would require a lot extra CPU to be managed. Specially considering than we are almost always having several mirrors in sight (other games could render them more nicely, but appear located in selected scenes where the strain can be reduced)
I would like a lot to see what you ask for. But as the game how it's now I really doubt they would add it because a minimal amount of players could benefit from it at all. Maybe the 1.40 lighting change will be bigger and deeper than I expect and SCS is going to add something like you ask (even if just partially). But I don't think its going to happen in the first stage of the lighting changes.
Who knows maybe they made me had to retire those words. I would be very happy to do so.