2 hp per cfm is a very solid N/A build, but if by "ideal conditions" you're referring to a maximum effort N/A build, well past 2 hp per cfm is routinely observed. For example, a PROPERLY ported W8 at 400 cfm, with the required supporting cast, will push well past the 800 hp level without breaking a sweat. 2.4-2.5 is obtainable, but requires serious refinement in every aspect of the engine.

With so many variables in play to maximize a particular combination, peak head flow being part of the overall package, is not the end all number in determining maximum power output. Not all cylinder head flow is created equal.