I had this same concern when I swapped my front brakes to a configuration I made with viper calipers and a 14" rotor. I run the mustang cobra rear discs. After some initial driving, the pedal felt great, but I thought I would have been able to lock them up on demand. After many continuous hard stops, I could feel them grabbing harder once I got some heat in the pads. I run a standard parts store pad, but I'm sure a more performance oriented compound would change braking performance greatly. Try doing repeated hard stops from around 50MPH. I bet they start grabbing more. Just like Mitch, I installed an adjustable prop valve for the rears and its wide open. There's no need for one. FWIW, locking brakes doesn't decrease your stopping distance.