Very true. Rough, but a accurate estimate of what can be done without going nuts. I see the 2hp per cfm math as what a really good street machine or bracket racer can produce themselves without going crazy on an engine.
It is not the ceiling.
Now the CID X 2 for HP is another story, but yet again, not the ceiling. Also, of course, what the head is capable of on the engine and its size is a constant variable to what can be done easy or with pushing it.
A 360 cubic inch engine at;
CID X’s 2 = 720 hp.
Cylinder head flow at;
220 x 2 = 440 hp
280 x 2 = 560 hp
300 x 2 = 600 hp
Etc....
While the cylinder head numbers suggest a possible certain amount of power possible, it would be easier to make the possible power on a larger engine rather than a smaller engine.