True, and they add a lot of heat to the air while doing it. I've been running a Jimmy on a street/strip car since '78. They look impressive, but they are a terribly inefficient way to generate boost.

In order to calculate the effect of boost on spring load, the pressure in the cylinder (the other variable in the pressure differential) has to be known . Obviously, the pressure in the cylinder varies dramatically during the engine's 720 degree cycle.
A simple boost x valve area calculation is only going to be correct when the cylinder is at atmospheric pressure, which is what the manifold pressure measurement is referenced to.

For a professional answer on the springs, Mike Jones at Jones Cams would be a good person to ask.