The Sonic we pretty much know everything about it so there is no actual prediction on the Sonic at all......
The current Malibu with the 2.4L DOHC VVT I-4 engine and 6 speed automatic returns 33MPG on the highway (has been tested as high as 39MPG).
So with the 2.5L directly injected unit and the 6 speed auto I don't expect fuel economy to drop 3MPG down to 30. I remember that it was projected to stay the same at 33MPG however I wouldn't be shocked if it picked up a MPG or two.
On the Corvette the V-6 turbo rumor has been going around for a long time now, in the 1980s there was actual work done on such an engine. Though currently these are just unfounded rumors which won't seem to die (much like the mid engine rumor). I am sure that whenever a new Corvette is in the planning stages they consider a great many things. From rear engine to a rotary engine....
Its possible that a I-4 engine may find its way under the Alpha Camaro (especially for European sales).
Would a 3,500 pound 280BHP 2.0L Turbo 4 be a bad thing?
Though I expect the V-6 may still be in it or even a turbo 6 as a option and the V-8 isn't going anywhere.