On a car with full exhaust and mufflers the difference would probably be less. The big tube headers probably don't evacuate as well once the system has some back pressure in it.
For a big street engine it is probably best to just depend on the torque curve to move the car. Pick a cam that has good, or good enough, drivability, and the power will end up where it ends up.
We would use our 2 inch dyno headers for a build like this and I'd expect more than 700 hp unless the cam was super small. But there can be a lot of difference between a production car headers and a really good dyno header.