There are two videos. In the first video the Ferrari 458 Spyder has the top down. In the second, the top is up. An interesting comparison showing the effect of aerodynamics on a top end roll race even with a car featuring as streamlined of a drop top design as the 458 Spyder.
The first run is a fairly close race but the 911 Turbo pulls decisively up top. In the second video where the top is up the 458 Spyder gets a bit of a jump which skews things considerably when trying to do such a comparison. The 911 Turbo still pulls but minimally. It is basically neck and neck with the edge going to the 911 Turbo.
Definitely an interesting comparison video and obviously when one is racing a convertible car, best to keep the top up for the best results.