Except that a decent blown gas C motor is making what, 1500-1800hp? That's 10 times more power than the flathead bangers we're playing with. I think the potential speed increase is scalable to the power you're working with.
Here's a perfect apples-to-apples comparison; the Kowalski crew ran a very trad looking A street roadster and went 120 with an n/a banger. Then they took the exact same motor and put it in their little belly tank lakester and went 140. Mind you they did this swap during Speed Week. When you're working with 100-150hp the speed gained by going from a brick to a dart isn't going to be as big.
Just sayin'...