DJ Qube

Assuming going from a stock brake setup to a Big Brake Kit would require me to get a 3mm spacer to clear the caliper, does that mean my rim / tire would extend outwards a total of only 3mm, or does a BBK rotor / hat setup have a "thicker" mounting depth than a stock one piece rotor, therefore pushing the rim / tire out even further?
Also, with the stock bolts, is there a general rule to how thick of a spacer you can run without needing extended bolts?