I have HPA's BBK for both front & rear. I absolutely love them and track my car 6 times a year. It's nice to have the larger brake kit for increased part longevity, extra heat capacity and less unsprung weight. Though as it has been pointed out earlier in the post a BBK should only be considered once you get to a point where you are hitting the limits of the OEM braking hardware or you are burning through pads and rotors like nobodies business from tracking so much

I have a PP GTI which has the same braking setup as stock Rs and it was more than adequate for learning to track it.
As for spare parts: the fronts use Corvette C5 sized pads, so any application which fits that will work in the front BBK at least. The rears work with pads designed for the 4th Gen Camaro. They sell replacement rotors and are very reasonably priced given they are 2-piece. I'm running Ferodo DS2500 up front and Baer ceramic pads in the rear currently. The calipers themselves are custom made from Baer, so I'm sure they have access to build kits (though I'd contact them directly for that kind of query).
If you wanna know more just ask!