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One thing to keep in mind are that most functioning spoilers don't create downforce, they kill lift. The retractable spoilers found on our 986s and Audi TTs are there for a reason, the factory did air tunnel tests and actual road tests, and found that they made the car more stable by killing rear-lift. The factory wants a car that has more lift at the front of the car than the rear, because that makes for a much more stable and forgiving car.
The software shown in this thread is amazing, but it doesn't replace air tunnel testing. If you were to see photos from air tunnel testing, the smoke plumes would show that there is a dramatic increase in turbulence around the top of the car with the top down. Turbulence = drag. My guess is that a 986 is faster with the top up than down, because of the increased drag. The top speed on my Miata was about 5 - 8 mph slower with the top down. The car shapes are different, but the top shape is pretty similar. A Boxster is about 30 mph + faster than a first generation Miata, so the drag differences between top up and top down would be even more dramatic.
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