As someone earlier said, there seems to be a greater effect on high HP trucks. I believe this is because the intake charge dynamics are entirely different for a normally aspirated engine versus forced induction. On a typical normally aspirated gas engine (say a 454 big block chevy), the cylinders are filled by the downward stroke of the engine with the intake valve open. Cylinder filling is greatly affected by the flow dynamics and efficiency of the intake manifold or intake runner. Cam dynamics and timing have a great effect on cylinder filling on these engines as well. On a supercharged or turbocharged engine, once at boost, the air is compressed and "waiting" for the intake valve to open. This pressurized charge is much less affected by imperfections in the flow-path of the intake manifold; so long as there is sufficient volume in the intake area to maintain some degree of storage. On high HP trucks, however, there may be a greater benefit because the engine is operating at a significantly greater level than originally designed, so the stock intake system may actually be a limitation to the inordinate amount of air required to support 600 hp worth of fuel. At these levels, you will see some benefit, IMO.
I also believe that a nice smooth intake would help in cylinder filling at low boost situations. The problem is, our engines don't spend much time off boost, so the felt effect will be small and short-lived, until boost builds, then the higher flow manifold will actually show less improvement.
These are just my ramblings, based on my experience. Others may have different views...