I am with JamesC here re: stock brakes. No doubt a high quality BBK will increase various aspects of braking performance (although I significantly doubt that it will decrease stopping distances much--assuming nothing is cooked). But to think that the stock setup is not good enough for HPDE participants is actually over the top IMO.
I say that as someone who ended up fading the stock setup and going off at WGI, but that was because of the HT10s self destructing and me doing too many hot laps in a row during long sessions. Barring such errors on my part ("bad" pad selection and over-pushing a street car), I could outbrake most cars I came up against in the advanced groups--including many lightweight stripped track cars.
I've been in bone stock E9X M3s driven by highly competent racers, and observed the same thing. So, this BBK emphasis is overblown IMO. Again, not to say that one would not benefit from a BBK, but one's money is probably better spent on attending more track days and improving driving skills rather than dropping $6k on a kit--unless $6k is of little financial consequence or you happen to be a really skilled driver who can really use the extra performance.
These rotors make a whole lot of sense to me. In theory, they should offer better cooling than OEM. Ducting would of course help a great deal. It is amazing that an M car weighing as much as it does ended up getting shipped without some kind of forced brake cooling.
If the objections are regarding pricing on bimmerworld's kit, I guess that is another story. I haven't worked through the numbers, but it sounds like it comes with F&R PFC pads, F&R SS lines, and fluid. The rotors seems to be $500 each. That sounds somewhat steep indeed. I think I paid $300 per OEM front rotor last summer after I went through the ones that came with car, which is a lot to begin with. These weren't available then. If they were ~$400, they'd be more attractive.