There are several disadavantage of doing this. If you plan to run single or parallel 18650 and double the voltage with a inductorless DC to DC doubler, then you will have to get one that can handle 2.5 to 3 amps output. Those high lumen light such as MS or Gemini requires 10+ watts of power consumption. First off, you will have to find a converter that can double voltage with just 3.7v input and output 7.4v @ 3amps. Something like that will add cost to the package. If you go inductor/capacitor, then it will be a bulky battery.
Another disadvantage is the logic of boosting it at the battery than having it buck in the driver inside the light head. Probably not worth the total loss of efficiency. The extra cost would probably be better used to have better quality battery.