20 watts carrier? I do believe it will do that but not be clean. Lots of IMD and there is not enough headroom left to fully modulate that carrier without major distortion. The finals are too small and nothing can change that.
I'm sure it is very clean from 4 watts. 4 watts is probably where a person would set it to make a clean You tube video.....
We're talking about a pair of 13n10 finals in this radio. Some manufactures like Stryker advertise 4 finals for this chassis but only 2 are finals, the others are drivers.
Just for comparison the 2879 is a 100 watt RF transistor. The datasheet shows lowest IMD is around 60 pep that's 15 watts carrier max in AM service to be as clean as possible. Do you think 2 13n10 mosfets that sell for pocket change are equal to a 2879?