I wouldn't say that the performance gains are limited to track, the kit comes with slightly more aggressive pads, I'm assuming a bigger disk/rotor and stainless steel braided lines with new fluid. All of which have an effect on stopping distance, be in direct or indirect.
As far as brake balance, lets look at the details of what drives those figures: Size of rotor, caliper piston diameter, pad coefficient of friction and tire, for the purposes of this discussion we can assume tire will stay the same and eliminate that from our list.
Most BBK will use a larger rotor than stock and to offset that increased torque, smaller caliper pistons are used to reduce clamping force (Pressure = Force/Area therefore, Force = Pressure * Area). We know that the 6 Pot system uses much smaller diameter pistons than the stock caliper, and it would make sense to me that the combined area of the 3 pistons (on one side) is at least the same as the area of the stock piston, so the effect of the bbk to brake bias should be minimal, especially since the brake line pressure will always be the same. If the bias is noticeably higher in front, you can compensate by using a more aggressive pad in the rear calipers, which is recommended practice.
All that being said, nothing wrong with stock brakes, but I like my brakes to feel and look more aggressive and BBK is an option for people like me.