Well i think the engine is limited to somewhat from 400 to 450 HP.

What else can you do:
- Reduce weight. Every pound that you have to move uses power.
I.e. leightweigt rims, carbon race seats (the stock seats are really heavy), use a carbon hood, remove all wasteful fluids like the water for the window washer, fill up the fuel tank only to the minimum you will use for the race, Remove everything in the interior that you don't really need ...
- Optimize grip. Use slicks, use a limited slip differential.
- Optimize the traction control (which is really bad in the XF)
- Remove glas with acrylic glas (Makrolon)

...