So I am a previous C class owner, and with that car I had gone through a few different rear diffusers. They came off easily from the rear bumper with a few screws and I could put a new one on pretty quickly. I was looking under the rear bumper of my new E63 and noticed that the rear diffuser is a part of the rear bumper and does not come off. If I go for a carbon fiber rear diffuser am I going to need a body shop to cut the rear bumper to fit the new one?

 

They screw on to the rear bumper.
Bumper has to come off.

 

If I understood your question correctly, the answer is... yeah.

http://extremedimensions.com/images/...3LookRear2.jpg

You yourself (or a body shop) will have to cut the lower center part of the OEM diffuser section (as shown in the link). The portion that's painted.

When i replaced my rear bumper, 4 Mercedes dealers told me the exact same thing... "there are no more E55 rear bumpers available in the west coast, east coast, and Germany. the closest thing we have is the E63 rear bumper."

So i bought that, cut and remove the center section and the rear diffuser fit perfectly.

 

So these CF rear diffusers require the bumper to be cut? I thought you just attached them with 3 M tape? That's the way I installed the one on my E55