+1 on just getting a RX8 front bar. It's cheap and easy!
...but jrj512 needs to remember that the majority of Miata drivers out there could care less about SCCA stock class autocrossing. Non-stock class mods won't affect your classing with BMWCCA, S2K and PCA locally. Some people just want to have fun with their cars and that's understandable.
Unless you're really serious about being competitive, just mod the car the way you want to drive it on the street. That's why I went ahead and did springs on the MX-5.