The september group is a much smaller group of mostly experienced autocross drivers - most with highly modified cars - roll cages, missing interior parts, modified suspensions, high hp engines, BIG brakes...and a few who live right by the dragon and drive it very often and very fast.
Bottom line - if you go, be careful and leave your ego at home. These guys will pull away from a stock svx in the hands of anyone but a highly skilled driver (aka...Porter) and you have to be able to let them. A parking lot full of cones would be a good place to visit often if you want to hang with these guys in the fall. I got lucky and had the voice of reason riding shotgun last fall - she probably kept me off the rail and out of the woods by screaming louder than my exhaust every time we got loose. I always thought I was a good driver but I got schooled by this group. Be careful.