When you widen your track like that in the front (i.e. with spacers) you increase the scrub radius, which may hurt steering feel and handling overall. I'm not sure how bad it would be on the 1er, but in general that is not desired.
I.e. see this:
http://www.popularhotrodding.com/tec.../photo_22.html
Your M3 control arms already help with this since they're slightly longer. This pushes the hub out, and thus the kingpin angle and axis out along with the tire itself. This might be enough to run a higher offset without hurting scrub radius too much.
I'm not entirely sure what the 1M does to combat this, since it has much lower offset wheels.
I would generally only recommend increasing track width (as measured from the center of the tire contact patch, not outside edge) after you've run out of space inboard. I.e. your tire or wheel touch the strut, or inner fender liner at full lock.
If you can actually make special control arms and whatnot you can do a better job, but that's some serious engineering work and not for a DIYer.