I own an 09 XF-S and I was recently bumped from behind. The damage is not major, but the rear bumper cover will need to be painted. It did crack the diffusor trim piece just above the right tailpipe. I'm thinking maybe this is a good chance to replace the diffusor with a tougher looking one. The picture is not my car, but my rear bumper/diffusor looks just like this.

Will a 2012+ diffusor fit an 09 bumper? I know Ill have to modify the bumper to receive the new tabs, but are the dimensions the same?

 

If you want the "tougher" diffuser off the 2012+ XFR then no it wont fit without a new valence as well. But the new valance is the same size and shape so it will fit.
Depending on where you get the new valance and diffuser from it may or may not go on easily, with some you need to cut extra slots in the bottom of the bumper.
See this vid: