Interesting, how did your pedal feel before the master booster change? I'm running 13 inch rotors also.
With the Cobra calipers and OE style booster/master the pedal felt OK in moderate braking. If I pushed harder it was like hitting a hard mechanical stop. I think I was just running out of assist. With the PB linkage ratio, bigger master and smaller calipers it just does not work well at all without assist.
Hydroboost is an option or the smaller master + stronger booster I used. In my experience, one or the other is needed.
