Hmm, I have the opposite experience; it flies very well on the included geared drive AND prop (make sure the prop is facing the right way), ample thrust to climb out and do outside loops, etc. I've also tried direct drive with a variety of props, and the results have been lack luster, launching is more difficult, takes time to get up to speed, etc. On 8 cell AA 1600mAh NiMh I have around 12min of flight time, and about 5 min on 700mAh AA Nicads 8 cell.

Don't put a larger prop on the included 1.8:1 GB, it'll over heat the motor; the included prop's about the correct selection. More speed, you can try 9 or 10 call 2/3A packs (HECell, KAN, or even KR600AE if you don't mind the shorter flight time). The other way is to go brushless.