I have the Whipple 3.3's on a 500efi .Gained a solid 18-20 mph. Its been flawless and made good power.Going on 4 seasons. I would buy it again!!! The new Quads or 5.0 should be just as good.
My HP 500 EFI Whipple motor made 903 ft/TQ and 839 HP
Hustler 500efi engine tear down & Build Up