Hi Gary,
See, I've turned in to a nit picker!
The SC is driven by a belt and pulley and there is absolutely no magic that at a given RPM the SC produces the same amount of boost *almost* regardless of load. For example, in a supercharged car at 4,000 rpm with 3/4 throttle, you're going to have the same amount of boost as in the same scenario at full throttle. I am conveniently assuming more throttle = more load.
In a turbo application, the above scenario is much different. The speed of the turbo varies with engine load. So, a turbocharged car - let's take an NSX, for example, can produce maximum boost levels at or possibly even below 4,000 rpm. If you cruise along in a turbo'd car and gently accelerate from 3,000 to 4,000 rpm, virtually no boost is produced. Do the same 3K-4K run at full throttle, and you can have full boost.
OK, back to the SC and to the example you cited. Virtually all SC applications employ a very simple boost bypass device (typically a Bosch part) that sits in the intake line between the SC outlet and the engine's throttle body. There is a traditional engine vacuum line that connects to this bypass valve. Vacuum from the engine will hold the valve open and as soon as vacuum goes to zero, the valve is closed (no bypass).
As a BBSC car, for example, sits at idle, the SC is still spinning pretty good creating a volume of air with nowhere to go. No worries, vacuum is high and so the bypass valve is open and the air just escapes out the bypass valve before the nearly-closed throttle body. Now, go for a drive in that BBSC car, mash the gas pedal and the vacuum goes to zero almost instantly... until boost starts to build at 3500 rpm or so. Keep that throttle mashed, and you'll see 6 psi at 8K RPM. Ooops, suddenly you see the black and white in the approacing intersection and you let off the gas pedal - still turning somewhere north of 7K RPM. The throttle plate slams shut, engine vacuum instantly returns and sucks the bypass valve open. The SC is still spinning like an SOB (with the same amount of work by the engine), but the compressed air is vented out the bypass valve.
OK, so there has to be a moral to the story. That moral is that all other things being equal - a turbo is a much better answer than a SC to the NSX HP deficiency woes. A turbo will give boost at lower RPMs (where it is needed) while simultaneously complementing the high-end performance.
If you size the SC pulley to give you 'x" amount of boost at 4000 rpm, you will have approximately '2X' (or more) at 8k rpm. This is a generalization (for all of the fellow nitpickers), but I use it to illustrate the concept. In current SC applications, if you want 4 psi at 4K rpm, you will likely have 10psi or more at redline which is usually undesireable.
However a properly designed turbo car can give you full boost way down low and still not blow out your top-end boost. The turbocharger, unlike the SC achieves this largely due to its characteristic responsivness to load.... which the SC doesn't have.
I hope that helps.