These are 19x10 ET25 on stock m-sport ride height and camber w/265s to give you some idea (on stock ride height, the 265s seem to work, but I will be dialing in the camber and doing a fender roll for safety's sake)...
http://www.e90post.com/forums/showthread.php?t=786191
...but et20 on a drop i think would be pushing it unless you really want to ramp up the negative camber and maybe allow some poke.