Bill:
This airbox modification was prompted, as I understand it, from a 6A that had to land in a corn field near St. Louis a few years back as a result of flying in a snow storm. Snow promptly plugged up the air inlet. Fortunately, a safe landing was made (go figure...nosewheel and all!). If such a modifcation was present at the time, it certainly would have "saved the day". It is important to note, however, that if carb heat had been applied once it was apparent that the pilot was flying in snow, the day would have been saved as well, since applying carb heat essentially closes the intake from outside air. Van's has stated that RV pilots have not been using the carb heat feature while flying in moisture properly.
As to the mod being mandatory, no. This is your airplane, and you are the manufacturer. YOU make the call here.
Regards,