Lee, the 15 HP gains does not necessarily mean it's the height of the plenum.
Lingenfelter was also getting 15HP but he went the other way (under) on big inch motors.
What this tells us is plenum volume is not enough no matter which way you go up or down.
Test this at the track (same day) do some runs with a nicely ported plenum then change out to this plenum,better yet i'd like to try it on my Z.
Then if it gains performance i'll just drive away with it.
Pete