Try this one: http://www.innovatemotorsports.com/products/dl_32.php

I'm not wealthy enough yet to afford such expensive toys, so I can’t recommend anything. I would try doing a search for a group forum that uses and is very familiar with the use of data loggers.
You need to record the engine timing, barometer, engine temp, air temp, axle speed, voltage, lateral G and forward aft G, fuel/air, rpm, fuel injector timing, and some other stuff I forget...

Also gonna need a 5 wire wideband oxygen sensor, unless the XF is already equipt with one?

Since your not street legal I would forget the piggy back computer, and get a fully dedicated system, that way it will be easier to interface with the logger and adjust the mapping cells for peak performance. I think the piggyback might even slow down data transfer during a quick millisecond adjustment at high rpm, I think it's only gonna get in the way of progress.

Have fun re-wiring your car! You'll know your making more power when that g meter is peaking higher and higher after every remapping, just try and keep a nice flat plateau on the G graph during acceleration.