The light's regulation makes a difference, but some now warn you as they draw down the cells, say with a few blink pattern, etc. That means the RCR's won't just go dark w/o warning anymore on a lot of lights.
I've also found that the amps can make the lights brighter, the way that the volt increases do.
For example, a light that can run on 2 RCR123's or one 18650 can be a LOT brighter on two IMR 18350...as the amps AND the volts are boosted.
If they are bucked of course, that doesn't work.
If I use one 3400 mah 18650, I get more run time than if I use two 1500 mah CR123's, and can typically support a higher amp draw, longer.