well, I'm still very much a noob, and never ran R-comps, but I'll give this a shot.
Basically your main cometitor in HS is the Mini. To be more competitive in stock class, basically you're gonna need R-comps and you will DEFINATELY need a competition alignment. All a cat back will do is let you hear what the engine is doing (which is helpful), but won't get much performance wise from it. Good struts would help too.
If you're just starting out, I'd run the car on street tires for a while just to get a feel of how the car acts at the limit, and have decent room for error. Once you can be quick on the street tires, you won't (generally) overdrive the R's.
That's about all I could think of off the top of my head. If you have any other Q's, please feel free to ask.