the 17500s shouldn't be used with the SF HOLA lamp. I mean, you *can,* but it will wear out the cells very quickly. monitor the cells regularly, test voltage going on and off charger to check for any irregularities. Test cells after they come off the charger, and then about 20 minutes later to insure they are holding above 4.00V and you should be ok. cells that won't *hold* above 4.00 V anymore should be disposed of, and using 17500s with a 2.7A load, will lead to them behaving poorly pretty fast. When they behave poorly is when they are most dangerous, please be careful....
The 2xC-li-ion will reduce the life of the lamp pretty dramatically in comparison.... a good 17500 will start off at about 3.7V at the load of an MN11, a very large cell, like a C size cell, could probably deliver upwards of around 3.9V into that same lamp.... this may not seem like much difference... but it's quite amazing really...
see... The MN11 was originally designed to operate at around 6.7V @ 2.5A on 3xCR123s. At that configuration, it is conservatively rated around 30 hours life. I tend to believe that on a bench supply at 6.7V, it would last closer to 50-100 hours, but they de-rate to accommodate for irregularities in cell performance, which can sometimes include "spikes" of power during initial turn-on.
Now having said that, you can see why your 17500s are driving it brighter, they are starting off hitting that lamp with about 7.4V, that's almost a 40% difference in light output. At 7.4V, the lamp should still have "10-15 hours" life in it or so... factor in the fact that voltage diminishes as you run it, and you haven't actually lost much life over running CR123s at all.....
But then step up to a huge cell, like the Cs.... You could probably hit that thing with 7.8V if you weren't careful.... By my calculations, the bulb should technically survive, but will have a noteworthy risk of instaflash on fresh heated cells. 18650s are a nice stopping point, most won't deliver any better than 3.8V into a SF HOLA load, which keeps you just on the other side of the instaflash fence IMO.