Yes, Power Plants have far more peak current ability than the wall does and yes, it is because of the onboard power storage.
Power Plants eliminate noise up to about 1kHz but let pass anything over that frequency, and area where the power supplies in the units they feed are most comfortable with removing. This is one of the reasons Power Plants sound so open and powerful, relative to a power filter that strips life off the music.
Your 450 watt amp would do great into a P20 even with the 15 amp circuit.