Will this disrupt other signals in the ISM band? If E2E efficiency is 60% there's maybe 30% radiating uncaptured? 400w is enough to jam anything else using the band for a long ways, maybe a kilometer?
No "erratic blasting" intended :) this is NOT like a strong WiFi.
We are using a strictly point to point collimated beam that is expected to meet nothing but clear air. It shuts down if/when any transient object is about to enter the beam path.
Also, don't forget that the important figure you should be looking at is power density rather than total power