Mods do move you up very quickly in the classes which is the major downfall to auto-x. Once you have any internal engine/tranny work (including cams) you are put into a VERY competitive class. My plan is to build my car as I originally planned, try to win SI class, and then just have a good time and not worry about beating the hardcore guys in the tougher classes.
http://www.scca.org/contentpage.aspx?content=61