The engine isn't gonna only see 256 cfm, it would only see that if it was pulling 1.5"hg at full throttle but it will be pulling vacuum way higher than that, air is compressible, it's just gonna use more pumping power to do so.
Sorry you're right they did test it @ 1.5"hg. Maybe they did that because everyone thinks in terms of 4 barrel cfm. Maybe I was a little too hasty in my calculation of hp from cfm. Need more coffee......
Agreed Air is indeed compressible and above 150 ft/sec the venturi coefficient of discharge is substantially constant.