My vw has an increases downpipe (2.5 over stock 2.0 inches),.cat delete and previous owner installed a magnaflow muffler , sounded better over all and i noticed no decrease in mpg, actually seems to have improved over all flow and may have increased mpg by about a mile per gallon ,I did get a code for emissions however with the removal of the cat
I assume that this would go the same for the xf aswell , just don't remove the cat you'll have a hard time passing emission tests without it