In general, real world 'street' users should NOT do only the front bar. Doing just one bar is about SCCA autocross rules, not doing things right. SCCA decided at some point in the distant past that for entry level 'Stock' (which is now 'Street') classes the rules would limit you to one sway bar choice, change front or rear but not both. Folks not racing just assume all the time that this one bar thing is the 'right' way to do things because so many in SCCA do that, but it is not about doing things balanced or right. In fact, if you do just one bar and nothing else the car is usually pretty badly unbalanced. Nonetheless, for autocross CS class the one bar rule means, as Christian suggests, that your best choice is doing the front bar upgrade for CS. This would leave you with serious understeer if you do only the front bar. The hole in the SCCA rules big enough to drive a truck through is with the bump stops. You cannot go longer, but you can go stiffer. This car is already using long stops as part of the spring setup, those long stops are a progressive spring setup as you lean harder and harder in the turns. So, for CS class we give you the big front bar upgrade and we balance that with stiffer rear stops relative to the front to 'rebalance' the car. That's why Christian notes harder stops for the rear....and softer front, fixing the balance issue you otherwise get with adding BIG front bar without upgrading rear sway bar. If not in SCCA CS, just upgrade both bars. The adjustable KONI also give us the ability to further dial the balance as needed.
On exhaust the Street category can indeed do cat-back. The ND Miata has a midpipe converter so ND owners can change only the muffler, on Fiat with just one converter under the turbo you are free in CS to run our entire RoadsterSport Complete System of Crosspipe, Midpipe, Muffler Delete.
On exhaust the Street category can indeed do cat-back. The ND Miata has a midpipe converter so ND owners can change only the muffler, on Fiat with just one converter under the turbo you are free in CS to run our entire RoadsterSport Complete System of Crosspipe, Midpipe, Muffler Delete.