You can't change out the CAT to a high-flow version and stay in the stock classes. Here's a summarized list of the allowable mods for the stock classes...
- Removal of spare tire and tools
- Front anti-roll bar(s)
- Suspension / wheel alignment using factory-approved adjustments
- High-performance DOT-approved tires (including R-compounds)
- Shock absorbers or struts (2 external adjustments maximum)
- Competition-type seat belts (no shoulder belts in open cars)
- Brake linings (pads / shoes)
- Air filter element (the “throw-away” part)
- “Cat”-back exhaust systems
- Wheels – standard diameter, width and offset (within ¼”)
- Roll bar / cage
- Gauges, indicator lights, etc.
Also, here's a link to the main SCCA Solo site and a doc that covers the allowable mods by class. It's not the official rule book, but that can also be down loaded from the SCCA Solo site.