A cheaper, easier option for a mild, yet noticeably improved brake upgrade would be to replicate the NB Sport brake package. Some later model NBs got larger stock brakes. Mazda just made extended brake caliper brackets to position the brake caliper further from the center of the wheel hub. This extended length allowed them to fit larger brake rotors and larger brake pads. Larger rotors means more surface area to dissipate heat, and more mass to absorb more heat. Bigger pads means they last longer.
The NB Sport caliper brackets fit on any 1990-2005 Miata. You simply reuse the stock brake calipers. All you need to complete the upgrade is the NB Sport package sized rotors and pads. I run this setup on my 1991 NA. DOT 4 fluid, NB caliper brackets, Centric rotors with StopTech pads. I regularly take the car to a low-speed track and have zero issues with braking performance. This setup performs exceptionally well, and is very cheap compared to any aftermarket brake caliper options.