There is no reason to check if %this exists in this case, as in order to call this function, you should be using %this.setMaxForwardSpeed(%x) , so if %this didn't exist, it would throw an error there.
Although it is possible to call it using Player::setMaxForwardSpeed(%this, %x); (exploiting how torque functions work), but if anyone is doing that, they should check if its an object themselves.