Pretty quick to see. Take a 4" cylinder bore, like the 302. About the biggest you can go with 2 valves is 2.02"/1.60." So your entire cylinder head area (if everything were flat) would be 12.56." The valves take up 3.205" + 2.011" = 5.216" of area. That's out of 12.56" of total area that could be utilized for the valves.
Now, if you were to do a 4 valve setup, like the Dominion 32 valve heads for the 5.0 OHV engine, it would use 2 - 1.60" intake and 2 - 1.40" exhaust valves. That's 2.011" + 2.011" + 1.539" + 1.539" = 7.10" of area. The increase in valve area to flow air through increased from 41.5% to 56.5%. An overall increase of 36% going from 2V to 4V per cylinder. Going to 5V per head, like a Ferrari would add even more, but it decreases in gain as you increase the number of valves. I think there are even some 6V heads out there for some engines. I can't see them being much more efficient than a 5V setup, and that's not a whole lot better than a 4V setup.
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