We're stuck with 14.7:1 fuel/air mixture. Big engines suck in more air which in turn needs more fuel. So displacement works against you. Therefore to get the best fuel economy you need smaller displacement. Turbo/blower engine would be one method, AFM the other. Either would need to be supplemented with reduced weight and better aerodynamics.
In either case, in part throttle it would run on reduced displacement, getting your 30 mpg. The turbo/blower effectively increases displacement by forcing in more air, so your 3.5 liter can be a 6 liter with boost. AFM of course just reduces/add cylinders.
I think GM should raise the price of Camaros (or at least the V8 models) to limit their numbers and play the CAFE averaging game that way. This would keep the cars at higher margins/quality/performance. Create a different fuel economy oriented car for the buyers that are about fuel economy eg Cruze SS, 130R etc.
BMW eventually went the turbo route. I hate AFM myself, so I would rather have a turbo v-6 or pay a gas guzzler tax when buying the v-8.