Anybody has idea if 87 or 89 gas is put into a Grand Prix GTP, what will happen?

You will have less protection from KR and Knock

Can be done, but cannot beat on car

Had to do it myself when the main fuel supply line busted in Phoenix a few months ago and all we could get was regular, if at all. Reduced power and mileage. More hesitations when I asked the car to put out.