Batteries what to consider when going from 4 cell to 6 cell batteries?

I fly a 680 hex with DJI naza using 2 4s lipos in parallel. I've heard 6sl lipos are a sweet spot lipo so other than verifying that all my equipment can work with 6s, which I have and it can, what other things should i consider? Is flying with 6s more efficient that 4s?
 
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