Quark AA v. 2x123 head w/ single Li-Ion cell

I know you can power either of the Quark heads with a single Li-Ion cell. Given the higher voltage requirements of the 2x123 head (as well as its buck-only regulation), what are the runtime/output differences, if any, of using a 17670 to power a 2x123 head versus the AA head? Most people seem to like the 2x123/17670 combination. If you happened to have a spare AA head lying around, would there be any advantage of popping it on the 2x123 body with a 17670? Wouldn't it run in regulation longer due to the buck-boost regulation of the AA head, whereas it seems you would be running out of regulation for most of the runtime of the 2x123 head? Wouldn't that buy a little extra runtime, presumably at the slight expense of some output? I guess one disadvantage of that setup would be that you rely on the cell's built-in protection to shut off at 3v, whereas 2x123 head won't run under 3v.

My head hurts...

Actually, this topic has been discussed in Part 5 of the Quarks thread. I had read about 3 1/2 parts of that multi-part thread before posting this question. I guess I should have kept on reading a little while longer. The answer to my question isn't quite as simple as I had hoped, but there's some good information about it in the first page or so of Part 5 of that thread.