It's not just the springs.
But.......it could be the springs, along with the converter and gears.

I've had one set of the sb rpm's in my shop that had the Hughes cnc porting.

Just an observation on my part, with no actual testing on an engine to see if it makes any difference or not.......but I don't care for the way they do the intake short turn on those heads.
It's as flat as a pancake.

Perhaps your cam and Gregs look somewhat similar on paper, but they are from fairly different lobe families, with yours being the more aggressive one, which would also tend to make it less rpm friendly.

Until you get some data(like from the chassis dyno), you're somewhat shooting in the dark.