The way It’s regulated, output stays the same. As for runtimes, we’ll wait for you to test yours and give us a report. 
Here is the reply from 4sevens just now…
*I see what you mean. The 16650 cells are not recommended either. The
light need the voltage of two cells to work best. You may gain lots of
capacity from the larger single cell, but the benefit won’t be more
than two rcr123, usually.
If you had the QP2A-X and a two 123 body, with a single 16650 cell,
this would work great.*
hehehe 
Why not!
I tried both, and I personally could not discern any observable difference in output, so I stuck with the 16650. I don’t like stacking cells if I don’t have to.
Sanyo 16650 as-is cells from FastTech. 2 X 16650 = $9.42
These are as is unprotected cells.
https://www.fasttech.com/products/1/10002357/1210700-as-is-sanyo-ur16650zt-1665017670-2100mah-37v-recha
FastTech is selling them as-is.
“These batteries are taken out of other industrial products and are sold AS-IS.
FastTech checks to make sure voltage is between 3.6V and 4.2V but we cannot make any other guarantees.
These batteries do not come with any warranty from FastTech.”
The provenance of these cells is honest, so if your looking for some budget Sanyo 16650 batteries, you might want to check these out.
No for batteries, I rather pay a premium to get some protected cells. I’ve never owned li-ions before and am cautious about them. intl-outdoor has them protected and its a bit more pricey but I can live with that for the added protection.
Thanks for all the awesome help!
Hi guys. Im bumping this thread as I need PROTECTED 16650 cells. fasttech does not have any as far as my searches tell me. Any tips where to get those?
Awesome! Now I need to wait next month to buy 
I spent way too much money this month. I’ll let you know guys how it works out
I faced this same issue as my old Pila 168s LiOns had reached the end of their serviceable life. Fresh new protected 17670 or Sanyo 16650s protected or not of unknown age, condition, and origin? Quite a few threads on this forum about them. Ultimately I decided to go with new product from a stateside dealer (Lighthound). I also replaced my well used Pila BC-2 charger with a new Xtar WP2 II unit. I’m pleased with my choices. This is not to say that I might not try some protected Sanyo 16650s in the future. Just no present need.