Denser colder air won't necessarily mean the map can deliver more power, but sustain that power for longer across the rev-band. But it would have to be a fairly substantial temperature difference between the morning and afternoon to make much of a difference to the results.
Most track days have a few sessions spread across the day, usually 20minute sessions unless its an open pit lane event. So he may get a couple of sessions in the morning and couple in the afternoon.
Try running one of the morning sessions stock, one with bluefin on. Then same in the afternoon. My guess is the ambient temperature won't make a huge difference, especially given our climate.
It would only be a measure of physical performance, not scientific. To measure power and torque differences during sessions, you would have to log the car for a few laps in the sessions using VAG COM and compare the results.