I guess the number of bars would be solely dependent on the number of diodes and their spacing on the board, no? I can't really understand how you could relate wattage to number of bars as something linear. That would be like me saying all 600W QB's should be 11" wide. That's not really the case, is it? Just sayin'.
More bars means more chips would be used. More chips means each chip carry less electricity, their lifespan could be longer than less chip design. In market, Most of suppliers would like to use more chips(more bars). Correct me if there is something wrong. Peace.