A formula one car will have between four and six times as much drag as a modern road car.
That's true. But the 400 attempt wasn't run in a normal trim. No rear wing either.
A closed wheel sports racer such as you will see at Le Mans will have lower drag than an F1 car but still a lot more drag than you would see on a production car as it still has to generate down-force.
LMP1 are around 0.45- 0.49 from what I can find. Which incidentally is the same as the next car....
If you want to see a competition car with lower drag than a production car, you need to go back to the late fifties or early sixties (D-type Jag, Lotus 11 or 23) before down-force became a thing.
A D Type is nowhere near a Prius, or even a GTR for that matter. Prius around 0.25? All I can find for a Lotus 11 is 0.34. I couldn't find anything for a Lotus 23.
Some old Citroens were really slippery interestingly.
A Veyron with 0.36 (0.41 normally), 736kW and 1888kg could do 407km/h
McLaren F1, 0.32, 461kW, 1138kg and good for 386km/h.
I'm not good at maths, but someone should be able to work out the power required to get to 500km/h
So again, put a big motor in a Prius.