Well, yes, this does come some three years after it was discussed. I just came across this and re-read its interesting subject. Just two things to close this:
First, as I mentioned somewhere above, this spoiler’s design does what an aircraft wing does, which is to create a pressure differential, only that the car needs the pressure differential in the opposite direction as compared to the aircraft wing. If you carefully inspect the XJS spoiler you will see that the bottom surface is rather curved from the front edge to the rear edge. Then, the top surface is rather flat with no curving. This means that, when the front edge of the spoiler splits the incoming ram air, the air flowing under it has a longer path over the curved surface than the air flowing above it, therefore it has to increase its speed in order to arrive on time to meet the over-the-spoiler air flow at the rear edge of the spoiler without creating a vacuum. The faster flowing air under the spoiler decreases the atmospheric pressure under it (Venturi effect), thus creating a pressure differential vs. the pressure above it. This is real. There is no doubt that the XJS rear spoiler meets above criteria having a longer air path under it than above it with the obvious consequence being that the air pressure above the spoiler will always be greater than the pressure under it, thus creating a differential pressure that causes “down force” and which will increase with the car’s speed. How much pressure differential does the spoiler create at, say, 150 mph? I do not know exactly, but I do know that it has to be greater than the down force on an XJS that has no rear spoiler.
Second, I do realize that cosmetics is a matter of personal taste, but, please, my BRG '94 convertible with that beautiful spoiler has no match, really, except for other XJS cars with that magic spoiler.