Mazda engineered the FD vent setup so that hot air from the oil coolers would not hit the brakes.

I guess they must have decided that the benefits were not significant enough to bother with when they designed the RX8. Many other high performance cars have setups similar to the RX8s also.

In my opinion venting through the fender liner like on the RX8 is probably better. I don't think it hurts braking performance and does allow for better airflow out of the oil coolers.