I did this a couple of years ago. I went with a company called V8 Race Experience. It was pretty good fun but 8 laps goes real fast.
The cars that i went in were just SS and XR8 road cars that had been race prepped. They had larger brakes, slicks, big clutch, exhaust, stripped out with cage, harness and seats. They arent scary fast but you would be surprised. You get up to about 200kph at AIR in Adelaide. I suppose it depends on the track as to how fast you go.
The grip was the thing that blew me away. I felt i was lapping fairly fast as we were catching alot of cars, however at no point did the car feel like it was at the edge of its grip levels like when you give a car on street tires a thrash. A lap with a pro would have been interesting.
Let us know what you thought after you have done it.
Cheers