First off gtshin...I'm claiming no expert status in flashlights & batteries, rather just a guy who's been tinkering with this stuff for a long time and spending way too much money in chasing new technology and buying a lot of flashlights.
I don't personally own the S10 or S20 but I'm unsure how you're arriving at a higher efficiency with the S10 over the S20, unless your planning on carrying a few extra CR123's with you and then running them in succession after each dies...which is fine, and might indeed give you a very marginal increase in battery life at the expense of inconvenience, e.g., battery dying on you in an extended emergency, needing high mode longer than anticipated, etc. At around $1 a piece on-line, the savings will be negligible. The only point I was making is that generally...a 2 X CR123 will give one roughly twice the run time as a single...and if an emergency arises on the road a dependable light with longer use "per charge" trumps efficiency from my perspective.
Safety Note: One BIG plus to utilizing a single CR123 as opposed to multiple lithium primaries is that if you're a person who's lax on swapping batteries at regular levels and/or not checking voltages, and tend to let them sit for years w/o use and then use them to exhaustion you run the risk of the more depleted of the two possible reversing polarity and causing a fire, explosion etc. Rare indeed, but it does happen and one needs to be aware of the risks of these chemistries or else stick with Ni-MH batteries and lights.
As I say, single CR123/AA EDC lights are really great little general purpose lights for throwing in a pocket for "non-emergency" uses, but the double CR123 S20 (from a practical standpoint) will likely give you roughly another 40% run-time at least per battery change-out. Selfbuilt, arguably the "de facto" premier on-line flashlight reviewer states that on CR123's, the S20 (XM-L2) gets around 2h 35m on high compared to the S10 (XM-L), 1h 12m on high...both to 50% depletion. *(PM me for links to his site if you wish to read more).