Yeah the D.O.T. approved is the only requirement for the Stock classes.
There is a difference between 'slicks' and the OEM tires on the car. Probably worth about 2 seconds on a course that long. But if you notice, many of these events are decided by .10ths of a second so that can make a HUGE difference.
Glad to see you did fairly well.
I run ROH 16 inch rims off of the WorldChallenge Contour. (used.. great deal

) They work great for me, the offset is perfect. No rubbing or other problems either. Another set of OEM rims (you can buy them new from Bat-inc. at a decent price) is probably your best choice. After that I'd go with kuhmo's. Everyone seems to love them and the hoosiers just don't last long enough. Although the hoosiers are the stickier tire.