one thing to remember, the farther away from the head port, the colder the exhaust gasses are, and the slower they become.
that's why a smaller tail pipe works. it has a tendency to keep the gas flow up.
however, i have no idea how much hp my engine will make, maybe 475 ?
above a certain hp amount, you need to keep pipe size constant.
let me know when you want to come over and bs.