Nope, efficacy. 経験値取得効率 is XP-Gain-Efficiency (if you want to look at in a dictionary), but I felt that efficacy (which 効率 can mean, as well as other things related to relative performance) fits better with the character presented here. It's the ability to achieve a result (viz. effectiveness) - and since the gals here have a specific goal in mind, efficiency doesn't fit as well.
