If your local club is run by the usual suspects (Miata, BMW, Corvette, Camaro, etc drivers) they probably don't know much about the "new" classes. So, when new people show up, and ask, the organizers don't really know where to put them, so they say "STS." Or, STX. I've seen it happen.
Get involved with your club, educate them, be the Street class expert. (An ideal, I know, but if not you, then someone needs to.)
In reply to:
I'll be at a disadvantage, and won't get any credit for being honest.
Well, not on earth, anyway.
Where does it end? You could get a 3.0 with hot cams, SVT flywheel, and maybe even an LSD and "no one would notice." The complete loss if integrity starts with a small step. Not knowing is one thing, but now that you know, (sorry).....
I don't know. You started this because you thought it'd be a kick to run your Contour. Well, is it or isn't it? You could run in G-stock if you replaced your airbox, and run on Victoracers. Fewer allowances, older class, so maybe less cheating--intentional or otherwise.
You could talk to your local club. It may be that, locally, they just plain allow more stuff to go on than the national rules allow.
In reply to:
why should I be one of
the few 'honest' ones...?
Because it has intrinsic value beyond the perceived short-term "benefit" of leveling the playing field in a weekend Walter Mitty fantasy.