The Airport apparently has a software system to detect adjacent-channel noise and select the channel with the least interference. Adding an external antenna after booting up defeats this system. You may see increased range, but at the expense of reliability. A better solution would be to reduce the power level to the least noisy S/R ratio.

This is not the case.

The white Airport Extreme basestation does not have such a "software system". The channel is set by the administrator.

This technique increases range, and in doing so makes it possible for wireless clients to connect to the basestation that would otherwise be out of range. Reliability is of course wholly dependent upon being able to connect to the basestation at all...

Reducing the power level might in some rather peculiar circumstances procure an improvement in reliability, but not in most.