most Zef designs range in the 20-40% power underating
For example xxk2500 are rated at 1000 bridged, been tested at over 1200rms. A KAR400.4 tested by carsound rated 50 x4, did 71rms x4...such and such, you get a lil more than rated. I know xxk2100's have same power rating as the KAR400.2, and they are good for ~120ish rms per ch, ~400 bridged.