What kind of brake upgrades are there for the rx-8? Will any other Mazda systems work like the rx-7?

 

the brakes are already excellent on this car. some upgraded pads & rotors (depending on your application track/street) and you will be good to go

 

+1

Two piece rotors are a nice upgrade because they save weight and run cooler. I have a nice set for sale.

After pads and rotors you could do front 4-piston aluminum calipers to save some weight.

No reason to go with a BBK unless you track the car hard with wide R-comps and 300+ RWHP. If you do go with a BBK, you need to do all 4 wheels, otherwise you are only hurting yourself.