I have a little (not a ton) of single rider experience, and those that I have experienced tend to follow the pattern I suggested. I guess my point is/was: all else being equal, would single rider load faster on RnRC or Test Track? In my experience it is clearly Test Track.
And I think the math/logic makes sense. Single riders get to load on TT with groups of 2, 4, or 5. On a configuration like RnRC, single riders only load with 3 or 5. Single riders load regardless if there's a group of 5, so there's no difference in those cases. But on one ride they load with 2/4, and another they only load with 3. While there are plenty of groups of 3, there are simply more total groups of 2 or 4 than groups of 3. So it stands to reason that single rider lines would move faster on rides with rows of 3 than rows of 2, assuming all else is equal.
All that being said, Hagrid's is a unique animal (er, "mythical creature"?). As another post alluded to, the guy behind us in line yesterday asked at the single rider cutoff if the preshow was included, and was told that the preshow was not, so he stayed in the regular line. Over time, tendencies like these will change and effect how single rider works.
If you think I'm wrong, that's fine, I'm just trying to explain my logic and reasoning. And most importantly, what I did experience yesterday is that people who arrived at the park the same time that I did and chose single rider at the cutoff point were in the exact same position as me. So if you think it will save you time, it probably won't in the near future. Though it would likely change in the long run.