I seen Darin Morgan say 2.5 hp per cfm is maximum, don't know how true that is or what it takes to get there, seen 3 engines do around the 2.35 hp per cfm.
In this video Chad says a 424 cfm head was good for 1025 hp on the dyno which is 2.41 hp per cfm.
As for the Big Mouths like to see a few engine making 750 hp and what it takes to do so if possible,
But don't think it's necessary impossible.