"The new sports car is heavier than "is usual" for a performance automobile...".
if you say, heavier than usual, then you are compairing the car in question.
if you say, heavier than it is usual, again, you are talking abt the new car.
if you cay, heavier than is usual, they you are sayin that new car is heavier than "what" is usual for a normal car.
did i make myself clear