I had a whipple charger on a 1998 4v Mustang cobra and it made 3psi boost at 2,000 rpm, 4 psi at 2,500, 5psi at 3,000, then 7 psi from 3,500 to 6,000 then 6 psi from 6,000 to 7,000.
In my experience that level of performance is somewhere between roots and cetrifugal. Back when I had it, guys with Vortech and Paxton kits with 7psi were making about 40 hp more than mine (I was getting 375hp at tires), but their peak power was around 6,800 rpm (fuel cutoff = 7,000) where my preak power was at 6,000. So the whipple design does fall off some at high rpm and you do get boost at any rpm (because it's positive displacement), but don't get full boost until you are at 50% or max rpm.
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