If you are limiting your options to only Mazda factory accessories in order to keep the car street legal; there are a wide variety of non-Mazda sourced add ons which would be street legal; but if the rules of the class in which you intend to race limit accessories to only those available from the factory; then there are very few options for the 3.0 motor. But have you considered installing the 2009/2010 3.7 liter V-6 that was used in the Mazda 6 and the CX-9 in those two years?
If you were to switch from the V-6 to the I-4; it would also require changing the transmission, exhaust system, computer, underhood wiring harness, and the in tank electric fuel pump (which costs between $400 and $625 for the 6I). Because of this; it would be far more practical to just trade your V-6 Mazda in on a 4 cylinder Mazda 6. The 6 Mazdaspeed has many special parts which are different from the 6I. So it would be best to trade for a 6 Mazdaspeed, for that reason.
But; if you are not limited to stock factory parts; then you can either install a supercharger or a turbo on the V-6; and still meet street legal specs for emissions. A machine shop that is experienced in building race motors could also rework the head porting, increase the compression ratio, and I expect someone out there sells cams which will still pass emissions, while raising the RPM of the power band. There are some experienced cam grinders who will make a one of a kind cam on special order; which will give you a big advantage over the folks who are afraid to alter factory specs. Dema Elgin; at Elgin Cams in Santa Rosa, CA is one of the most highly qualified people in the country to do this. I have had him build several one of a kind cams for vehicles I have owned; and they consistently ran much better than the original parts.
But please bear in mind that any modifications which significantly increase performance potential will inevitably need custom tuning
with the help of a dynamometer and exhaust gas analyzer; which usually at least requires recalibrating the ignition advance and the fuel injection mapping; and could also involve intake and exhaust system design changes, increased fuel pump pressure, and adding stronger ignition. They will also probably require using a different heat range spark plug than the stock item. If you are not willing to do this type of tuning; which can become expensive and time consuming; then your car will not be competitive. Custom dyno tuning will make any car faster; whether or not it is modified.