My guess is somewhere around 5-10 hp? Maybe? That's just completely shot in the dark speculation though.
As far as the huge drop when running regular fuel, I don't really see any big deal with it. Generally speaking, turbo cars have frequently required premium fuel anyway. My (speculation, again) is that it was detuned to run on regular fuel to appeal to more mass market buyers that may not care as much about peak power.
Welcome to mass production of vehicles designed to appeal to the lowest possible common denominator of buyers.