I know you said, "Do not mention anything about a Geo Metro..." but I think this is applicable.
In the mid-'80s, Suzuki built a turbocharged 3 cylinder. The US got it in the Sprint Turbo. Canadians continued to be able to get it in the Pontiac Firefly.
I mention it only because it was able to achieve similar horsepower and torque numbers to Ford's n/a carbureted 2.3 Lima, early carbureted 2.2 Chryslers, and GM's 2.0 OHV and 2.5 Tech IV.
It was pretty low tech. It had things like a gundrilled camshaft but it was still just a two-valve head and was using '80s "land of the lag" turbocharger technology.
If GM could put a 2.5 in a car the size of a Lumina, I wonder if the 1.0 turbo could have powered that car? It was only about 10 hp and ft-lbs less than the "Low-TechIV" at similar rpms and weighed considerably less than the cast-iron four.
Of course they would have had to engineer a transmission for the little 'Zook with proper durability and ratios. The little 3cyl was undoubtedly more expensive to produce than the 2.5 was (GM was only paying about $600 to produce the 2.5. Materials, casting, machining, manpower....all about $600.) So it wasn't financially feasible. I understand that. And fuel was still real cheap so there wouldn't have been any reason to. It's just a question of, "What if?...where would we be today?"
IIRC, the early Luminas had a really low CD. Very aerodynamic. A six passenger car getting 45+ mpg freeway?
maybe....