The Prius using more fuel than the M3 when driven at the same speed - it's still surprising, even though clearly they hammered the Prius while the M3 was puttering behind slowly. A bit of a ridiculous test.
The other thing they said though does make you pause to think, and I quite liked it: Rather than selling your fuel guzzling vehicle now, it's quite possibly much better for the environment if you keep driving it, but drive it slowly. Making a brand new car has a huge environmental impact, so large that the difference in fuel consumption would take a pretty long time to overcome it. In the case of a Prius it's even more complicated as the batteries are particularly environmentally unfriendly - and there's a lot of those in a Hybrid.
But that was a silly and contrived test, which had absolutely no bearing on the real-world fuel efficiency of the car. If you're choosing a car to drive around a track all day, by all means, avoid the Prius.
I do love Top Gear, though, and Tesla should have known better than to hand their car to them.
They actually kind of did for Toyota Prius here: http://www.youtube.com/watch?v=dKTOyiKLARk