Your 502 MPI should make at least 650-725 horsepower with that State II. If you're hitting the rev limiter which is 5050 RPM with a 30" prop, you have to be going much faster than 77 MPH. If you do the math you should be at around 82-86 MPH with approx. 10-14% prop slip. If you're hitting the rev limiter, the problem would be in the prop, not the Procharger. If you can spin a 30" prop at 5000 RPM, you must be going faster than 77. The way you figure it is like this:
Prop Pitch X RPM divided by the Gear Ratio X 1056. That shows you your theoretical speed with no prop slip. Then you take that Theo speed - your actual speed and divide that number by your actual speed. That gives you your prop slip which for your boat should be 10-15%.
I have a 25' tunnel with a 502 & a Stage I Procharger. Before the Procharger, my top speed was 70 with a 26" Bravo I prop. After the Procharger and a little experimenting with props, my top speed is 94.5 on GPS with a 28" Bravo I that has been lab finished and bent to a 29.5"
Be sure you're not running too rich. Check the fuel pressure with a manuel gauge hooked to the Procharger fuel pressure regulator. Be sure you have at least 50-55 lbs, but be sure it's not way high. That Stage II pumps fuel into both ends of the fuel rail and if the regulator is set too high, it will foul plugs, etc. - Hope you get it straightened out. I think it's a prop problem.