S12's are good, but don't forget the dunlops. I personally like to mix and match, with an s12 rear and a dunlop 756 front.
Re the rear brake, it's easy to change the height of the lever, but the current level is close to what the pros say is a good starting point. At least if you use the semics foot position advice - keep the balls of your feet on the pegs, unless you are rear braking or shifting, or bottoming out the suspension on a jump landing or g-out. When you want to brake or shift, lift your foot, then move it back when you are done... Sounds ungainly, but it works :). The problem with lowering your lever is that you won't be able to reach it when you are in the usual braking position - standing up and leaning back. Unless you have crap boots and a double jointed ankle that is :D .