dougmc;
What I usually look at are single cell lights like you, and running them in a series doesn't increase runtime. The only difference between the 18650 and 26650 is as you can already tell is that one cell has a larger diameter, hence larger volume so it can have a larger capacity. I wouldn't be surprised if you can use a 18650 cell on 26650 lights but you would need to have a spacer to keep the battery centered and rattling. Runtimes wouldn't be that great since the capacity of 16850 is a 25 percent less than a 26650. If you don't run the lights on full and carry extra cells I would just think you can turn the lights up for more lumens than choosing to add more lights to the bar/helmet. It's going to be the simpler solution IMO than spending a whole lot on a new fancy light.
The maelstorm series is exactly why I am considering switching to a 26650 cell
4sevens usually makes a quality light. I am thinking the S12 is more of an ideal light for a single light solution since it has higher lumen output and flood, it should make me more annoying/visible to cars at night.
X10 is more of a thrower, but I guess if you have a multiple light setup it doesn't matter. But then again the X10 does last longer, and is the cheaper light. I have also seen a few charts showing the difference between the two is less than ideal but I guess that's real world performance.
Either way I think it'll cost 150~ due to needing new batteries. So I'm not going to jump on them just yet.