I rattle can'd some M Sport side skirts on my non M Sport 328 just this past weekend. To echo what others have stated, prep is key. I just used a scuff pad as opposed to sandpaper to rough them up, rinsed with water and cleaned with an APC. I can recommend
https://www.automotivetouchup.com for a good paint mix, if you dont mind spending the money for the proper color match. Their paint matched pretty darn good (Glacier Silver Metallic). The most difficult part for me, was getting a good high gloss finish. The skirts came out looking a bit matte in comparison to the rest of the car. I think the key to higher gloss finish is more light coats versus fewer medium coats.