318 exhaust size

A dual 2 inch is closer to a factory dual exhaust. Minor gains. But they are there.

A dual 2-1/4 is fine for minor engine upgrades. The most modest cam should be employed at max.

A dual 2-1/2 should be the standard exhaust size with or without headers. It is an excellent size for power and decent sound control for street or street strip vehicles. You should be able to go very fast with a 2-1/2 exhaust.

A 3 inch exhaust should be used on a heavy hitter. N02, turbo/supercharged engines where there at least 85% track. On the street, they tend to be loud! A pain to fit vs. a 2-1/2 exhaust though a few here do run it. I just find it a hard pressed reason to use a 3 inch exhaust on something like a street strip ride where it is in the street 80% of the time.

Can you find useable power in a 3 over a 2-1/2 for street vehicles? Sure! But is it really necessary?