Niall,
You are not saving weight really when going 3s. See chart on this thread.
http://www.oople.com/forums/showthread.php?t=37230
When you go lower voltage you get worse mileage so you must now have a higher capacity or mah pack to compensate thus nullifying your weight savings of that extra cell. Go 4s in my opinion with a lower C rating (especially if your concerned about crazy power) such as 25c. It will be cheaper, lighter than 40c and your motor and esc will run much much much cooler than a 3s 40c battery set up.
another thing to watch if you are watching the car calories is how much Mah your really need and get the minimum plus 10-15% and the biggest thing that effects your mileage is how you drive. If you drive your car like a gas car with all the bleeping of the throttle they normally do you will get piss poor mileage and if you are too light and use too low of a gear you will have massive Mah sucking wheel spin as well.