A lot of cars running 15's daily, I would not worry about brake cooling unless you are really working them hard on a hot day. You can add some defectors to help, Porsche ones have been used, DIY pretty easy to fab up, etc.....
I am using the middle raised area of the bumper cover which is sealed below with a splitter/undertray combo, ducts mated to it, works really well, barely any added weight.
I am using the middle raised area of the bumper cover which is sealed below with a splitter/undertray combo, ducts mated to it, works really well, barely any added weight.