Head flow requirements will be dependent on the ci displacement of the engine being boosted.
Forcing air into a cylinder rather than pulling it in with pistons is more efficient but ultimately there is a limit. To make 1200whp you are going to need some efficient heads. Trends Engine Analyzer states that a 500 cid engine need approx. 400 cfm intake at .700 to make 1900 crank hp or 1300whp. A smaller motor is much more forgiving to lower flowing heads which may be what you are seeing. Just look at the Mustang crowd and there 341ci engines making insane power.