Surely with the double clutching and such to downshift to first there is a very significant advantage to being able to stay in second on a road course and especially autocross? (I haven't driven my Z much since I got it so correct me if wrong:) )
I think Kenny did show that the straight line racing differences are fairly marginal (based on shift times) so this may be a real world reason.
I really like the comparison tool but a straight line race doesn't consider all the reasons why people choose a 4.10 diff.