You will very definitely degrade performance in going from 3s to 2s (Given the same prop, voltage of 7.4v instead of 11.1v will result in far lower amp draw, and thus power since W = V x I). You could increase prop diameter and draw a far higher current (if the motor could cope with it) and end up with the same power-in [but I doubt the motor would be happy]. In choosing a battery you need to look at its capability for delivering what the motor/prop will demand - for higher current draw you need larger capacity (mAh) or higher C rating. Look at the max amp/max watts for your motor... do not exceed either of them. You can choose whatever voltage you want, but to get the maximum power out of your motor you need to optimize both voltage and amp draw (suited to the motor Kv and the size of prop you wish to use, to get the correct balance of speed and thrust).