650 or 750 Edelbrock on 340

Both the 340 & 440 engines with the 6-pack set up make 15 hp more than the 4bbl. One has a 625 cfm 4 bbl, other has a 750 4bbl....but still 15 hp difference for each.
The 6-pack intakes are true high rise intakes [ compared to the 4 bbl ]. Smoother bends, less restriction. So how much of the 15 hp increase is due to this?