RE: Not impressed with Great Planes Ultimate EP.....
The only thing you're going to save by going to a 20C battery is a small amount of weight, MAYBE.
All things being equal, the 25C battery will spin the motor faster, due to its ability to hold a higher voltage under load. Odds are going to the 20C, performance will be worse.
Without changing anything else, the best way to get more power is a larger, or higher-pitched prop. The key is to not go too big, or you will overload the motor, ESC, and/or battery.
After that it's a matter of whether you want to buy a new motor or a new battery. A motor with a higher Kv rating will spin the prop faster and provide more power.
Alternatively, a higher-voltage (i.e. 4S) battery will also provide an increase in power. However, it will also increase weight more than a motor upgrade, and will require you to add an external BEC to the system.