Had been looking into this off and on for a while and jumped on a set of M3 calipers when I had the chance to. As far as cost-effectiveness, I would most likely not go this route if I had to do it again, and instead would just upgrade to the 330i front brakes since they are a comparable size rotor to the M3's while costing much less and do not need any machining work to fit them. I'm still glad I decided to do it, though, since it is a cool part to have on the car and will definitely add some bite to the brakes. Thanks for watching!