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So how much better fuel economy do you want ? My C7 gets 29 mpg on the highway today with a NA 6.2L in V4 mode. Even around town, I am getting over 20 mpg. Don't get me wrong, turbocharged small engines work pretty well for most passenger cars - I've got a Volvo V60 with a 2.0L turbo four and it makes over 240 HP and 258 ft-lb. of torque. Coupled with an 8-speed automatic transmission, it does pretty well and gets 35 mpg, but it is not a Corvette.
Normally aspirated engines are simpler than turbo engines, have better, more linear throttle response, and can run over 100,000 miles between major overhauls. (Try that with a Ferrari or a McLaren or a Lambo.) NA engines are also are more predictable at the limit - no sledgehammer of torque at the wrong time to put you into the guardrail.
Further, Mercury Racing / Marine has shown that with the addition of DOHC heads, the Chevy small block can compete with anything out their, including "supercar" smaller V-6's and V-8's with turbos and / or superchargers. The only issue with these heads is that they make the engine wider.
Another solution for more power is to put two small block V-8's in series - then you can make over 1300 HP and still have an engine that weighs less than 900 lb.s - and run over 100,000 miles without the need for an overhaul. The offshore superboat world has already invented such as beast - see below. It would fit nicely in a mid-engine Corvette.
Double LS3 OHV Cam-in-Block Superboat Engine
The 2017 NSX with a 500 HP small 3.5L V6 with twin turbos gets 21 city/22 highway(CAFE).
My 427(7L) NA Z06 with 505 HP is rated 15 city/24 highway(CAFE). In the real world I average 29 on long highway trips and 33.0 MPG on short runs(like a 450 mile cruise from Springfield, MO to Natchez, MS with near perfect conditions for great gas mileage), and there is no V4 mode on my Z06. My combined average runs around 24 MPG.
Forced induction engines have less compression than NA engines, thus they are not as efficient when running low RPM's while cruising down the highway, and not under boost.
My 4 cylinder 2.3L daily driver has a factory Roots blower with low compression. It weights 3250 pounds(my Z06 weighs 3175), and my DD has a .32 Cd while my Z06 has .34 Cd. My DD gets real world 30.5 MPG on the highway(less than what my Z06 gets under the same driving conditions).
Of course, there is no comparison in regards to performance between my DD and my Z06.
The Mercedes AMG GT has a 4L DOHC V8 with twin turbos on top (with it's wider heads than the Chevy small block OHV engine's heads) and it fits under the hood(and without a tall "power bulge" raising the height of the hood). It can be done with proper engineering.
If the car is going to be priced over $100K, forget about a V6, clever marketing machines will convince people a V6 is the way of the future, nonsense...
