Well in series is going to give you twice the voltage so there it is. Double the voltage will make a bigger differrence than the C or the mah, but it sounds like you are getting ready to fry some electronics. Are you sure you are ready for that? The other thing is what are you looking for when you say performance? If you want it to run twice as long then go for the parallel wiring.
www.battlefieldraceway.com Racing videos at http://www.youtube.com/snoopyrc