I am not sure of the answer as I have not done that much testing with a stock cylinder head and cam. Your point, I assume, is that if the stock exhaust port is the principal restriction in the exhaust system, then we can't get more exhaust flow and power than the exhaust port will support merely by playing with the header and the rest of the exhaust system. Assuming that is true, there may still be advantages based on tuning with the flow we can get. The ITB cars with essentially the stock exhaust port are getting 150 HP or more at the crank with 4 -1 headers, though not necessarily in a power band appropriate for the street, so it looks to me like improvements in power are possible up to that level with the right exhaust system.

Lets see what the testing shows.

John
V-performance.com