ok, as you can tell by my awesomely crappy paint pictures, alcohol and anger dont mix, lol

but, with the undertray on. you'll have more drag, due to the fact that the undertray prevents air from having a fast escape path, it hits the back of the firewall and is effectively squeezed through the opening at the back of the hood (top line) and the gap between the undertray and the firewall. this is a big cause of the excessive underhood temps as the air has no place to go and basically slows down before it leaves the engine bay

without the tray, the air has a less restrictive path once it passes through the radiator and enters the engine bay. It quickly finds its way out of the engine bay and most of it will pass under the car. This of course causes more lift but at the speeds your average xf owner travels, this means nothing. If it were a true track car, it would matter. But by lowering your car you can negate this since more air traveling over vs. under = more downforce.

With the undertray my car ran at or above the center of the coolant temp gauge, without it it always runs below, no matter what im doing