Regulators and BECs expect to see certain voltage differential to work efficiently.

I've recounted in other threads how Giant Scale flyers I know were using 7.4 v packs regulated down to 6v, and were discovering that their operating times were extremely low.

Switching to an 11.1v pack of the same capacity quadrupled the operating times.

All too often people make the mistake of trying to regulate the voltage down very close to what the battery pack already puts out. This is not a good idea with many of the simple circuits used on these things.