The Audiopipe amp is efficient, have you used one and not "heard it from bros and the internet?" It is definitely more efficient than an USA-2000X and they make about the same amount of power.
Saying a BIG amp is a power hog is like saying a 572 big block chevy V8 is a gas guzzler, you need Sherlock Holmes to tell you that? Two 800+ CCA batteries, one run of 1/0 gauge power wire and a 120+ amp alternator is MINIMUM requirement for a 2000+ watt RMS amp, if you have anything less than that then you don't have the right to complain about efficiency vs. power output because your electrical system isn't up to the task in the first place.