Do not forget you loose HP to the wheels because the fins produce rotational aerodynamic drag. This will reduce your MPG, produce more pollution,
blah, blah, blah. Get the new dress for your wife instead.
Not too sure about the loss of energy, quite hard to measure I can say, having spent many hours at Lucas Girling as a graduate trainee on the dyno experimenting with various disc brake cooling designs. We only focused on cooling efficiency and noise, as there was no significant kW absorbed power difference measured on the drive motor.
However it's surprisingly easy to overheat drum brakes, and once they get hot braking capability is severely impacted and it's hard to lose that heat without stopping and letting them cool down (if you can of course). Apart from work on the dyno, I'd previously proved this to myself while still a student in a 1970's Mini 998 with drums all round coming down the bank from Buxton towards Leek and trying to stop for a stationary car turning right...
Anyone using a sporty car with drum brakes in a hilly area, or who relies on the brakes working at full capacity all the time, should definitely consider Alfins.