With an 8T pinion, flat blades, and 3S Lipo, it is actually MUCH easier to fly than with the stock setup. It is much smoother, and the tails wanders around less since there is abundant power for both motors at all times.

In my opinion, as soon as you can routinely hover the stock battery pack dead, you should upgrade. With an 8T pinion, and a Thunder Power 3S 1320mah, I am getting 25+ minute flight times, and the heli behaves much better.