The centrifugal supercharger essentially the compressor side of a turbo, but driven by the crankshaft rather than an exhaust turbine. The mechanics by which they compress air and maintain manifold pressure on a running engine is the same.

Yes, there will be differences in characteristics of efficiency because of the differences in what drives the compressor (one system may get a different amount of net power with the same amount of boost), but the differences cannot possibly be so much that the supercharger is able to produce 40% gains with less than 1 psi, while the turbo does not produce any significantly noticeable gain at the same amount of boost. In general, I would expect the turbo to be more efficient at creating net gains from boost pressure, because it uses some "free" heat/expansion energy from the exhaust.

Your example of the Whipple producing 50% gains at low rpm... that's with more than 5 times the amount of boost at which RIPP claims 40% gains! With a similar boostower-gain efficiency, the RIPP would make less than 10% gains at the rpm/boost in question. That seems a lot more believable.