When I wrote into street machines i asked them the exact same question and this is what the had to say
"It's already an expensive exercise for Ford to convert the Mustang for
Australia. Bolting on a blower and re-engineering the Stang to cope with the
power AND making it reliable enough for warranty AND achieving Aussie
emission standards is a bit 'out there' for such a short run (100-unit)
import. It would be much cheaper just to install the blower yourself and
hope the rest of the Yank eningeering will stand up to the added power and
torque."
I knew I didn't delete it from my inbox for a reason