In a car as heavy and fast as a mustang, I don't think you can have too much braking ability. I opted for the BBK on my car because I always wanted Brembo brakes, felt that bigger brakes would be better than smaller, and I may take the car on a road course at some point in time.
I would say to go for it. I am curious to know whether the master cylinder on the BBK cars is different than that of the regular cars. If it is, personally, I'd rather go all in than just part of the way by buying the caliper/rotors, but not the MC. However, if the MC is the same between setups, sounds like the OP is getting a decent deal on some good brakes.