I haven't seen any carefully documented fuel economy gains from just adding a turbo. Generally, the automaker's turbo/efficiency approach as you say is through downsizing the engine, and/or going with numerically lower gearing.
On the gearing: I actually swapped in a taller (numerically smaller) final drive ratio in my 1.0L Metro/Firefly. Fuel economy improved 5% at 80 km/h (~50 mph). Acceleration suffered, but I don't mind. Theoretically, I could have put a small turbo on it to re-gain the acceleration, and when staying out of boost also preserve the MPG gain.
Mitsu could offer a turbo 1.0 with taller gearing than the 1.2 which would outperform the 1.2 in both acceleration & fuel economy.
(Then the speed fanatics could swap their taller trans with the shorter 1.2L economy fanatics trans and everybody's happy. )
It's fun to speculate.