I have the plain tan carpeting on the rear shelf of my 98 550, so I don't have any direct experience.
I think the "Lusso" diamond pattern is a great look but one of the things I've heard, when I was shopping for my car, is to be careful of the color used.
My understanding is some of the colors reflect a lot on the back glass and it can be distracting. So, it might be worth it to first sit in a 550 that has the leather rear shelf in a similar color to what you're thinking of putting in.
Please post some before & after pics and lessons learned when you get it done. Good luck with the project.