Here are a few pictures of my stock 135i with M Sport Package after installing BMS spacers - 15mm front and 12mm rear. I took her for a walk after installing the spacers last weekend and there were no signs of rubbing. I think the front is as wide as I could go stock, and while there might be some room for the rear to be bigger, I'm not sure it wouldn't rub even moving just up to 15mm.
Hope this helps!
Cheers,
Bryan