My feeling is this: you'll get every penny in the BBS package back when you sell it. Heck, you can get MOST of the money by selling the wheels on Miata.net and running cheaper standard wheels.
With proper pads and fluid, you shouldn't need the Brembo package for raw stopping ability OR heat dispersion during a track session. HOWEVER... the pedal feel is much better and more consistent with the fixed calipers. There's a real difference in the pedal feedback with Brembo and with sliders. So in your case I'd pull the trigger and get the package.