Yes, it needs a deceleration in excess of 15 ft./sec. to activate the brake force display - that is, when a briefcase goes flying off the seat, or your dog in the back thumps into your headrest.
Wouldn't there be a safer way to check its functioning?
Such as, you in the other car pull off onto the shoulder, and have the X3 come by at the speed limit and brake sharply just after it passes you.
But then it's up to the X3 driver to time things so that there isn't someone anywhere in their rear view mirror.