I have 100 cell cats on my XJR. 200 cells would be a great improvement too. But they will throw the P0420 and P0430 codes and check engine light, which needs to be corrected in the tune.

The factory cats are a "double brick" with a 600 cell in front and a 400 cell behind, just to give you an idea of the difference an aftermarket cat can make in flow.

The exhaust will get louder with higher-flowing cats, the factory cats also act like a muffler to reduce the exhaust noise, so bear that in mind if you are removing other mufflers from your exhaust.

There are a couple of vendors here on the forum who are doing bolt-in cats. I just got my exhaust bloke to cut out the old ones and weld in the replacements.

 

Ok. So I better plan to do the cats and the tune at the same time. Anyone have any recommendations or experience with the aftermarket CATs for the XF's. I guess I will go dig around the forum and see what I can find.