I'm completely stock except with a catback exhaust.

 

Google is your friend!

According to http://www.moutons.org/sccasolo/ you are in A Stock:

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Stock

As a rough guide, if your car has modifications beyond this list it is not eligible for Stock Category:

* Anything available on the exact model and year of your car as standard or optional equipment, as installed on the factory assembly line (nothing available only dealer-installed is allowed), plus
* Standard OEM identical replacement parts
* Accessories, gauges, appearance, comfort, and convenience items (not replacement driver's seat, though) that basically don't help
* Added roll bar or roll cage to spec (not required, though)
* Driver harness (no cutting seats to install, though)
* Trailer hitch and/or tow bar mountings
* Any wheel of diameter and width identical to a stock or factory optional wheel, offset within +- 0.25 inch
* Most street legal tires that will fit the mounted wheels and stock bodywork
* Any shock absorber of the same type and mounting as original; no change to standard spring mountings is allowed; suspension geometry may not be altered; gas or hydraulic shocks are permitted
* Any brake linings; pre-1992 cars may use solid/braided metal brake lines.
* Any front anti-roll bar (no change to stock/optional rear anti-roll bar, if present)
* Manufacturer specified ignition settings only
* Any replacement air filter element (or removal of air filter element); no plumbing changes, however
* Engine bored no more than .020"; no balancing or porting/polishing of head
* Replacement of any part of the exhaust system past the catalytic converter (if quiet)
* Any oil filter
* Added clutch scattershield

 

Correct, you're still in Stock Class (AS for our cars).

Cat-back is fine, along with front sway bar, r-compound tires, and shocks, not coilovers though. Those are the main things to do in the stock class, with the tires obviously having the biggest effect on going faster.