318 Difference?
302's would raise compression because they are close chamber heads. Just let me tell you, the difference between a 8:4 to 1 motor and an 8:9 to 1 motor would not be felt by the seat of your pants. Flow is much more important than compression. In other words, an 8.4 to 1 318 with a 4 bbl, 340 cam, 340 heads and headers will demolish a 9:5 to 1 318 2bbl with single exhaust.
Wow.
Lets start with your first point.
The goal of building an engine is to try to tweak every last bit of power out of it that you can. the "seat of my pants" cannot accurately tell the difference. A dyno or quarter mile slip can more accurately.
If you count your pennies, your dollars will always be accounted for....
Flow AND compression are needed for a good engine. You have to go for BOTH, not one or the other.
Your example with a "8.4 to 1 318 with a 4 bbl, 340 cam, 340 heads and headers will demolish a 9:5 to 1 318 2bbl with single exhaust" is like comparing apples and oranges.
Comparing a single exhaust engine with a dual exhaust engine is not a fair comparison. The single exhaust on a 318 is the bottle neck for the engine. I had a 70 Swinger with a stock 318 2bbl. I did an acceleration test from 70 to 100 MPH and it took a long time to get to 100 MPH with the single exhaust.
I replaced the exhaust with a mandrel bent 2 1/4" dual exhaust with an h-pipe, and the car went from 70 to 100 MPH in almost 1/3 of the distance that it took to do that with a single exhaust.
It is a given to upgrade to a dual exhaust.
We are trying to help him pick the best components for his engine to achieve its best potential and giving him tips for improvement.
Compression is efficiency. The higher compression will have more efficiency, but you don't want to get too high where you have to run premium or racing gas to keep it from knocking/detonating.
