A supercharger isn't that common of an aftermarket addition to a Porsche 928. I would guess that having a non-stock engine would limit the desirability for an investment. If the installation was done well, the supercharger can be removed and the engine returned to stock configuration.
Rich
Membership Chairman
Charter Member
1993 928 GTS Cover Girl
1987 928 S4
1979 928 5-speed rescue
1979 928 5-liter track beast