RE: Concept X64 stock battery
More C = More weight.
However more C is safer.
So it is better to target or use a battery that provides excess "C" capacity, but doesn't go too far.
If your going to add more weight, you are better off making it USABLE weight and going for a higher mAh rating instead.
That gives you BOTH a higher effective "C" e.g. a 2800mAH 20C is effectively better than a 2200mAh 25C in terms of power draw AND it will give you longer flight times, for about the same weight.