depends on what pulleys you have. i think sc12's run out of puff at 13000rpm (supercharger rpm). mine running 15psi pulled hard until 7200rpm (engine rpm). full boost from 4000rpm (engine rpm).
these rotrex spin up to 35000rpm (supercharger rpm) or some shit. that means theyre working maybe 1/3 of their potential when on a 4ag regardless of engine rpm.
$2750US is a bit steep for a s/c though.
im a fan of superchargers over turbos but its so much cheaper to get a good turbo setup than a good supercharger setup.