My Mi5 comes in at 40 grams overweight with my full sized packs, and 60 under with a shorty. It allowed me to go with a full sized servo, esc in the rear passenger side, and use lead to corner balance the car. The shorty is till 4600mah, and we run 6 minute races at my track, and I come off the track only about .1 volts lower than the full sized pack guys. Our traction is really high, so the lighter car is better... If you can get down to minimum weight with a full 6000+mah pack, then that is great. My car is a tad too heavy with them, and at my track, laptimes are certainly better in lighter trim.
Everybody will have a different opinion on this, and the traction levels, weight of your car without a battery, and nature of your track will dictate whether a heavier pack with more MaH, or a lighter pack with less MaH, will provide better lap times.