Opti-Coat won't keep brake dust off the wheels. It's just another layer that the dust will sit on. You have to reach the problem at its core, which are the terrible stock pads. Get aftermarket pads that dust much, much less than the stock pads and this problem goes away:
http://zeckhausen.com/BMW/E92_M3.htm
Look for the "Centric Posi Quiet Ceramic pads", that's what I use on my car and never worry about excessive dusting.