BR's just drop the "on" power to 6-8v. I think it might have something to do with burning out points quicker at 12v.
The starting advantage comes from the 12v direct "start" power, which is only for a few seconds.
Yeah mate, they're bosch and would have to be one of the most commonly used coils around.
The failure would be due to people using a GT40 WITH a BR, or a GT40R WITHOUT a BR.
Not much that I'm aware of. Maybe you can take note of some specs of each, output voltage etc.