Here are my belated thoughts on this discussion.
I think the best potential in using turbos to reduce fuel consumption is that you can move the same car with a smaller displacement engine with similar performance to the larger engine. There would be weight savings which would help improve city mileage and on the highway you would use less fuel if you stay off boost.
I was surprised at how much complexity a turbo added when I started to really get into understanding and improving the power in my Miata. There is a lot of additional plumbing, an intercooler, additional oil cooling capacity, bigger radiator, blowoff valve, catch can and boost controller. Most of this stuff is fairly light, but it must be costly to manufacture.
A turbo could be maximized for fuel efficiency by reducing the amount of time you are under boost. I drove a CX7 and it seemed to be under boost constantly even under light acceleration. Perhaps that explains it's higher than expected fuel consumption. My 04 Miata gets slightly better mileage than my 99 did because boost isn't a factor until just under 3k rpm. I have changed the intake, exhaust, boost controller, and blowoff valve to gain an extra 50 whp, but the fuel economy is down because boost comes on much earlier (2k rpm +/-) and it is on boost quite a bit more even under light throttle.
It will be interesting how Ford handles turbos.