noob ahead:

interesting read. im on my first set of XP8's all around and i have not experienced the initial bite problem that you described. i put 100 miles of track use on mine and they are still brand new. yes, it was my first track day, but i felt that i was pretty hard on them by mid-day (learning the insane bite versus stock pads, i broke much much later).

again, admittedly, i am new to this, but the XP8's grabbed pretty linear to me, until the pedal was deep to the floor where it seemed to get close to locking up (is this what you're talking about?) my rotors are also fine?