As a former BMW service advisor for several years and BMW parts guy for the last seven, I’ve noticed that BMWs on performance tires often only get 12-16000 miles without autocross wear.
That said, BMWs are often camber challenged, and understeer at lower (autocross) speeds, which is hard on fronts. Higher pressures will help, and camber plates will also help. New autocrossers tend to charge the corners hard and exacerbate the understeer issue. The phrase goes something like “Slow in, fast out. Fast in, out backwards”. That would be the part where the fronts finally find grip and the back is hung out to dry.
Tires with really small tread blocks tend to have more of a chunking problem. The individual blocks squirm around and tear themselves apart, especially on the outside edges of the fronts.
I would look into tires with larger tread blocks (I really like the Dunlop Direzza Z1 Star Specs), some negative camber and higher pressures (possibly over 40 psi) for autocrossing. I would guess that your camber is around zero to 1 degree positive and your car may have toe in. Up to -2 degrees is fairly friendly on the street and just a little toe in (maybe 1/8″) Other E9X autocrossers can help you with more precise numbers.
Then, take it a little easier going into the corners, try to get more of your turning done early (also known as late apexing) and accelerate out of the corner. If a street tire is squealing (high pitch), that’s okay, but as the pitch deepens into more of a squalling noise, the tire is being abused.