With a 25% increase in displacement you'll get an equally big hit in the mileage numbers.
It would probably be a 10% hit, at worst. A 2.5L Skyactive wouldn't have to work as hard - like how my 400hp 5.7L Caddy can get 26 on the highway while my 1.6L Civic gets 31. It's not a linear scale is all I'm saying.
Mazda already tried offering a more powerful engine in the CX7 and not enough people bought one but when they offered the naturally aspired 2.5 in the CX7 the sales climbed.
They also dropped the price.
This will not play well with a turbo. A turbo restricts flow out of the cylinder (the exhaust backpressure drives the turbo) causing knock, so you loose some of the high compression ratio advantage. Maybe they could put a blower(supercharger) on the G and drive it with an electric motor combined with the larger alternator/capacitor used for braking that Mazda has developed for electrical accessories.
A turbo wouldn't work well on a street driven 13:1 compression engine, but they could probably do 11:1 with a turbo (premium fuel). The turbo increases dynamic compression and therefore efficiency.