2 1/2 inch exhaust on a 318...

I have personally seen the 360 heads make 580 horsepower with potential to make up to 600. The port job ran about 800-1000 dollars on it... port match, polished, new and bigger valves and a few other things. But I do agree with the Edelbrocks. I dont know to much about the MP big port heads so I can not comment on them. So let me get this straight.... 400-500 will do good with 2 1/2 inch pipe and over 500 should be 3 inch pipe? Thanks all!