Another plus to upsizing the countershaft sprocket is the possibility of increased chain life. Less angular bending as it rounds the sprocket. I believe that below 17T is where this angularity really starts to affect chain life. The 500 Bullet was designed in the 1950's to be maintenance friendly, it comes with a 17T countershaft stock. Of course, general cleanliness, proper lubrication & adjustment are likely of more practical import than a theoretical 17T limit, but it couldn't hurt.