Awesome reviews and feedback, I rarely see F30's at the track so it's refreshing to read this. While not the same car, my S4 is likewise a fat pig and the tires get scrubbed pretty hard while trying to get it around the corners. I definitely know that no tracks are created equal, but I took my PSS out to gingerman raceway, which is pretty nasty on tires/brakes. I got tons of sidewall wear, but no chunking like you are seeing. So I agree that you are probably overdriving and/or taking a line you are used to from a different car? But I too felt that the PSS were greasy about mid session, with hot pressure at 40 psi. For reference I ran 255/35R19 PSS on 19x9.5 wheels, which is a pretty decent stretch.
I decided to switch to a dedicated track set and have 2 DE's on some Dunlap Direzza ZII's, which are MUCH better than the PSS over the entire course of the session. They held up to some pretty hot/humid conditions at mid-ohio this past weekend. They are pretty cheap since the new ZII SS is coming out, so tirerack has ZII's on fire sale.
Regarding the brakes, I'm a carbotech xp20 guy myself, but the pagid's are just as well respected from everything I've heard/read. The F30 comes with a nice 4 piston fixed caliper, so I wonder about the benefits of a BBK. Would it be possible to run a proper set of 2-piece properly veined rotors? Something like Girodisc or Stoptech aero rotors? Everything I've read says that the stock blanks are not directionally veined, which obviously makes a HUGE difference in heat dissipation. From this
link it would appear that FL/FR rotors have the same part number, implying they are not veined properly. I personally don't see how going to a large rotor without switching proportioning is a good thing. You'll push the bias forward and ultimately reduce the overall stopping performance (you could argue this is marginal of course).