A new more powerful motor will require more power from the batteries, which increases the weight, so you end up chasing your tail trying to improve performance.

The plane flies with the included packs, but they really need to be cycled several times to obtain full power.

A computer controlled charger ( a must for RC ) let's you see how much current the battery is holding and is capable of discharging.

Often you can also use a slightly higher voltage batter to get improved power... a 9.6v pack would do wonders, and a LiPo may also work as well.