Personally I'm not a fan of the "FBI Cant".
For the fastest draw remember you want the tip of the barrel to travel the least distance to a straight line to the BG. Adding another 15 degrees of travel makes that draw to a straight line that much longer. I know we are talking about fractions of a second, but sometimes that can make a difference. It also depends on your method of draw... many people actually push down to get a grip before they pull up and out. Others simply draw with fingers beneath the grip. It's all a matter of personal comfort and training. While I don't prefer the 'FBI' cant, it's better than nothing and certainly workable for most people and situations, and it does make a comfort difference when driving.