If you're really close to the track, it generally means your nose it parallel or slightly leaning toward the ground, meaning consequently a stronger initial "boost" for a following BR. Pulling the nose up obviously means slowing down, and that would mean starting almost from an inertia point for a BR. The momentum wouldn't be the same as being close to the track. This said, i dont think height affects the power of a BR but the speed at which you're travelling before performing a BR. Well at least this is how i feel it I may be wrong!