If you are getting nearly 4000 mah out of a gp 3300, then you probably were one of the lucky people who recieved the famous gp 3700's in 3300 shrink wrap. they sent quite a few of these out the door. a good gp 3300 should only get 3300-3500 mah worth of charge. Also, this # is not a true mah reading, because it will fluctuate with the ir of your cells. If the packs are in poor shape, they will appear to take lots of mah, but will dump quite quickly.
I've always found that just running the cells in race conditions is the best way to tell how they perform. especially with a brushless motor as you can eliminate the motor from the equation..