Hi there gents, let me introduce myself. I recently purchased an XK 2007 in indigo blue with 30 k miles on the clock. Absolutely love it. I also own a 1977 Triumph Stag which I painstakingly restored to more or les mint condition.
Now my old Stag has the original 30 ltr V8 and the sound from the exhaust is very nice as it's almost a straight through exhaust.
Now I got to thinking that the standard exhaust on the XK really did not do the car justice so I purchest a full S/S system with the twin tail pipes.
I am at work at the moment abroad so I got my garage friend to install it. It went on with very little problem, however there is a problem where the tail pipes clash or are very tight to the rear diffuser and the bumper arperture.
I have looked at replacement diffusers and they are very very expensive so I can't see that I will go down this route. I have thought of completely removing the diffuser to give plenty space for the twin pipes.
Do you guys have any ideas what myb options may be. I have even considered having the tail pipes changed out for smaller ones, however they are welded to the system and would require a significant amount of refabrication. I bought this S/S exhaust second hand as it was almost new and in great condition. Not sure who made it but it is a quality piece of work. Looks like a Paramount set up though.

 

This is the XF section. You should probably post this in the XK section. You'd get more info from those that own your car.

If it were me I'd have the exhaust modified to fit the diffuser. That should be easier/cheaper than changing the diffuser out.