I don't think anyone discussed this or not, but I think something else to consider is wheel/tire combo. I purchased my X with the BBK and before I got my rims, the X stopped really well w/ little fade under moderate driving (~4-6 heavy braking from 60-80 mph). After I upgraded my wheels/tires, they show much more fade. I think the drums are adequate enough for the stock rims if you upgrade to BBK up front. But if you go bigger w/ wheels/tires (I went w/ 20s and 28.2 overall diameter, 1'' greater than stock), the drums in the rear most def can't handle it. Also, I you go staggered like I did, I believe that more braking power can now be applied to the rear (bigger footprint) and not to mention more rotational force, creating more heat, thus needing more surface area (ie bigger brakes) to stop and to prevent fade. When I have more $$$ after my suspension upgrades, I will upgrade the rears as well. Hope this helps.
NOTE - I didn't know they had an option for 6 piston brakes, if I would have known, I would have gotten those, then maybe I wouldn't need to upgrade the rear. Just something else to consider.