As far as I know, Loic Bigois did not
have a car design role at Honda-Brawn-Mercedes.
His role was to oversee aero technology. He was not responsible for the shape of the car, but rather the aero technology used to shape the car.
Bigois' former role has been expanded to include Vehicle Dynamics and is now headed up by Geoff Willis.
Often confused, but many teams have separate roles for aerodynamic technology
and aerodynamic design
(the shape of the car).
(E.g. John Iley, who used to be head of aero at McLaren, was not responsible for the shape of the car but for the development of aero technologies at the team).
The leading roles responsible for aerodynamic design
are the Chief Designer John Owen (who was promoted from Principal (Chief) Aerodynamicist to Chief Designer last year) and Technical Director Bob Bell to whom he reports. I think Owen combined the roles of Chief Designer and Chief Aerodynamicist until Mike Elliott arrived. It is likely that Mike Elliot now reports to him.
Edit: I think Owen now reports to Aldo Costa
I would not point to one person being responsible for either success or failure in the design of F1 cars.
Strategic decisions about the design direction and design are not made by one person and it's all about having the right people, in the right roles in the right structure.
Mercedes are clearly not there, yet.
Edited by Timstr11, 01 August 2012 - 11:10.