This is much more complicated an issue than what some may sound it to be. There is a relationship between cfm and bhp. This is however depending on a huge amount of factors such as, to state a few examples, volumetric efficiency, brake specific fuel consumption, engine volume, rpm of maximum power output et.c.

As far as heads concerned, just making a head flow 400 cfm sure as hell wont guarantee to make it flow well in an engine. The problem is to make the head flow well through all stages of cam lift, air velocity, air density and humidity. Now thats black art. It doesnt boil down to total amount of flow when it comes to horsepower, it boils down to volumetric efficiency.