I'll bet Derek (HDDP) can respond with the question about sequential gearboxes in SCCA events, post the question in the "Racing" section and perhaps you'll get your answer.

FYI, on the issue of Sequential gearboxes. If you race/drive primarily road courses, where you're shifting constantly, using both brakes and gears for slowing the car, hitting the "right" gear is crucial, missing a shift (wrong selection) can be disastrous. With sequential gear boxes, you always know which gear you're going to get "next" (up or down). As "Former" pointed out, Nascar doesn't use sequential boxes, because they're not shifting all that often and, other than the road courses (2) they're not really shifting (usually) on the track, mostly as they are entering, exiting the pits.

HOWEVER, do you really want a sequential gearbox on a street car for daily driving. Well, unless you're obsessed with driving, this could get to be a real pain in the @ss in everyday conditions.

As I said, ask Derek about the reason behind the SCCA issue.