911 Carrera: 2.9 liter twin turbo flat six, 400 horsepower
911 Carrera S: 3.8 liter twin turbo flat six, 530 horsepower
911 Turbo/Turbo S: One million horsepower.
Though chances are that the GT3 will remain naturally aspirated.
I think that if this happens, you can expect the Boxster/Cayman S to get the 2.9 liter turbo engine, and the base Boxster/Cayman will get something in the 2.5 liter range.