This already bolts right up to the sedan as the base plate has been modified.
Here is one of the threads that I used to retro fit it onto my sedan:
From my understanding the M3 mirrors 'probably' bolts right up for the coupe non-m, assuming the pillar is the same as the M3's, hence why the base plate doesn't need to be modified if I am thinking correctly. Because the base plate has been shaved down a little for sedan fitment, you may see a bit of a gap. It fit perfectly on my car, I don't have a close up but here's how they look on my ex-car. If you want it to fit, then maybe you want to find out if you can use your existing non-m base plate and see if it can mount up to the mirrors.