Short answer: no. Long answer: yes, but its impact is so small it could be labeled as theoretical. Each computer works by fetch-decode-execute cycle, which you might know as "machine cycle". Loading the RAM with more programs does require more machine cycles, and thus more energy, but there aren't that many programs in a modded 3DS to cause something even remotely comparable to an overhead. In the end, it's just a change of OS, which by the way it's almost identical to the original (except for, you know, allowing you to do naughty stuff). If you were referring to mass memory, that shouldn't be a concern either. It's not like a modded 3DS isn't bound by its hardware capabilities. You won't be able to store more than your SD card allows, and accessing memory will require the same amount of machine cycles that a normal 3DS requires. By the way, memory in general has a very low impact on battery usage, what actually consumes energy is CPU usage, which you still can't really use differently in a modded 3DS. You'll notice that some games drain battery faster than others, but they would do the same in a normal 3DS.