Eurocharged is the tune of choice, they had a $200 sale a few weeks ago, don't know if it is still in effect.
The other two things for an NA ( non-supercharged ) XF is a 74MM throttle body and a CAI ( Cold Air Intake ).
There are posts on debaffling the current intake setup, if you don't want to go the CAI ( $$$$ ) route.

You can also install a Sprint Booster which makes you think you have more HP, but really just makes the throttle more responsive.