On my E39 I took some 1/2 thick weather stripping and put it around the flange of the speaker. On reasembly its pushing out slightly on the panel, you dont even notice and it keeps the panel away from the moving speaker. I have a feeling the same will be needed in the E53.

I went with the Hi-Energy HSK 163, its a 3 way system similar to what the X has stock. I was going to use them in my e39 but the Two way system does well. I had both the C5's and Focal Utopia's in my E39 and removed both to put in the Hi-Energys. For the X5 I didnt even bother, just decided to use the Hertz stuff.

Good luck!