There is a guy in the track section who had similar issues with the z rotor kit. I would replace the rotors first before getting a full bbk.
Racing Brake,
DBA, and
stop-tech offer two piece, bolt-on solutions that seem to hold up better to track abuse than the z1 rotors. Interestingly, they are all a pound or two heavier than the z1 rotors. Could be something to that...