You need to look at this from the standpoint of the manufacturers, I guess. These breakers typically cost them $3-4 to manufacture. I can guarantee that each of the breakers has not been tested (too costly), so they might test one in 100 or one in 1000 (batch testing). Whatever the batch size might be. BTW this same factory test scenario is no different than when a larger, much more costly breaker is manufactured (200-4000A range). I have been invoved in a number of facilities where new breakers failed during testing at rates from 10-25% due to poor tolerances.
Typically, we do not test small breakers (15-100A) with direct current injection. It is just too costly. However, we perform thermographic inspections of distribution equipment where we find overloaded breakers, loose connections, and corroded connections all of the time. Many times we identify circuit breakers with overloads via thermographic inspections and they might have only 5-10A on them, due to poor contact resistance.
I could tell you stories about how many of our customers have not performed maintenance on their distribution facilities for 20-30 years. During testing, we might find a number of protective devices inoperative. You know what we call a circuit breaker without overcurrent protection? A = A switch
So believe me, the last thing you want to do is to overload these things. Being an enginee, I can not in good conscience, tell anyone to load equipment past recommended levels.
P.S. Most manufacturers try to compete with breakers and panels at the residential level. I have not found a particular brand which is significantly better or worse than another. (Square D, Siemens, Cutler Hammer, GE etc)