This is basically what the 1M has underneath it's rear end. But the 1M has a hybrid driave shaft... M3 in the rear half, and 135i in the front half. How will you bolt up the M3 drive shaft>?
Also... I think you will have to flare your rear fenders because your rear wheels will be sitting further out (~2cm) plus, wider track.
You get a +1 in my book for going ahead with this project!
PS: stick with the OE M3 subframe bushings. They work just fine for the street and track. I have them in my 135i.