I've been shooting the 640 a number of times since my original post with the B/C gap question.
I'm happy to report that the gun works fine. Quite accurate with a variety of .357 ammunition and no spitting or any other indication that it has a larger than normal gap.
I'm satisfied with it and it's become a favorite carry gun.
I found this old article mentioned by K22fan on the effects of the gap on velocity very interesting.
http://dwcollectors.org/dwca-content...un_article.PDF