I think the m-sport really benefits from the m-perf rear diffuser. The standard m-sport rear diffuser is the most plain of all the F30 lines, imho. I think BMW recommends it in conjunction with the front lip spoiler to maintain aero balance as well.
It's a port-installed option, or there's significantly cheaper replicas available off eBay.
BTW, the m-sport brakes will anthacize your wheels for free with brake dust in a couple weeks
