Have not seen any tests on the efficiency issue. I agree on the duct routing issue, specifically to cool the caliper and pads. Back in the day when we went to disks on a roundy-round car, we preferred to mount the caliper forward to get an air duct to it without having to go through the engine compartment.
Noted from the Bentley Continental Speed and an AMG SL65 in the garage at work, the front calipers are mounted low and to the rear. This seems to expose at least part of the caliper to the air stream under the car. Both cars have a little flap underneath that looks like it would direct air to the caliper.