2x, 1.5x?...Not sure what you mean.
the 650cfm is through the suction or the discharge of a MAF or equivalent?
I assume the cfm is measured at suction side where you meaure your 15"?

oem intake system (stock filters) - infers the standard 2 x pipes....so 325cfm each - yeh?

if that gives you 15inWG - you can still square rule it down to 650 or 450.

------------------
couldn't quite find an easy comparison of "same car same day" stock v others - sorry.

------------
I did pickup a previous comment that there is more "slip" past the vanes at low revs. and less intake restriction helps minimise this, still small fry unless it all adds up to a bag of chips...;>)
ie: 2% + 1% + 2% + 1% + 2% + 1% = 9% improvement overall.