Back in the mid 80s I think that it was Road and Track magazine got a current NASCAR GN car and an IMSA sports car at the Ohio test track, which is a 7 mile banked oval. The "stocker" was Tim Richmond's Chevy Monte Carlo and the "sports car" was Al Holbert's 962 turbo Porsche.
To make a long story short, Holbert screwed the boost knob through the floor and went 238, (and blew the motor!) Richmond went 256! Of course they did fill all of the windows and lowered the Monte Carlo to about 1 inch ride height and I think that they ran a 389 cu. in. "qualifying" motor but still if you remember the mid 80s Monte Carlos they were really bricks! The sporty car guys went home with their tails between their legs.
Around 1980 I was at Daytona for the IMSA Paul Revere 250 which ran at the same time as the July Firecracker 400 and Peter Greg, the IMSA "hot shoe" at the time, was going to show the Nascar guys how to go fast around Daytona! He had a Porsche 935 turbo (a 911 with a sloped nose and a big wing) and it was probably making 750+ hp, he had Moon wheel covers, lowered to the ground, the wing was trimmed to almost flat and he went 196 and sacred the holly sh-t out of himself. Cale Yarbourgh was on the pole for the 400 at around 198!! Again remember that the "stock" cars of that period looked like bricks.
It would be fun to see a 2005 Cup car with the unrestricted engine go for top speed, if they could make a brick go 256 the latest models could really be impressive.
Rex