First of all, power isn't the point, it's the capacity, which in the case of the example, 2700 mah is the capacity, not the power. Power is usually expressed as watts. Anyway, most people put higher capacity packs to get more life or more flights per flying day out of the pack.

The so-called "brownout" you are talking about was a problem that was mainly caused by operating the receivers with battery packs that were running low. The voltage threshold for the receiver was fairly high, but lower than normal operating voltages most of us used. Spektrum changed the "time to reboot" the receiver so that if it did go into a cycle, it would take less time for it to recover. Most of us overcame that by using 6 volt packs so that the battery voltage never drops low enough to cause the "brownout".

CGr.