You're supposed to use piezometer rings to measure the pressure around the pipe, but a simple tap with a pressure gauge is good enough.

For a horsepower reference, the 2013-14 GT500 made 662 hp out of a supercharged 351" V8. It has 2.75" exhaust to the cats, and 2.5" pipe to the mufflers mounted at the back of the car. The Hellcat is the same- 2.75" exhaust, 707 hp from less than 400 cubes.