Here is a picture of my '18 540 M-Sport with a gloss black diffuser.
I believe it is M-Performance carbon fibre styled, but in gloss black. Got it from ebay because I can't justify paying three times the price for OEM equivalent.
Fitment is 9/10, very impressed.
Finish overall is very good, 8/10 because there are some rough patches of paint on the underside of the diffuser, we'll see how the gloss black finish withstands harsh Canadian winters.
Poor quality pics in my garage, will take better pics of my car once the rain ceases.
Installation is pretty straight forward, undo the 8mm screws at each bottom corner of the OEM diffuser, undo the 5 or so 8mm screws in the middle of the underside and then yank it out. The clips holding it in place will let go, start from the bottom left corner and work your way to the right side.
To install the new diffuser you just have to get it to clip into place and put the screws back in. If you have a hard time, try bending the clips up a little it'll make it easier for them to catch on to the bumper.
Feel free if you have any questions !