I look at the ATS (hopefully the same underpinnings as the 6th-gen Camaro)...the specs show:
2.5 I4 (202 hp)...3315 lbs...22/33 mpg
2.0 T4 (272 hp)...3373 lbs...20/30 mpg
3.6 V6 (321 hp)...3461 lbs...18/26 mpg
Now - it is worth pointing out that the heavier V6 Camaro 2LS (designed expressly for maximizing fuel economy) has a mpg rating of 19/30. I don't know why there's such a large difference. But IMO...that could be taken to mean that Cadillac didn't squeeze as many miles per gallon out as they could have being luxury oriented, and that their V6 model is supposed to be performance, while ours (Camaro) is more for economy. Perhaps then, the 6th-gen Camaro (using the same engines and similar weights) COULD get 2-3mpg higher than a similarly-equipped ATS?
All speculation at this point - but as #3 pointed out, I think it's very clear that the tried-and-true GM 2.0L turbo 4 is more efficient than the high performance 3.6L V6.
By the way....where did this idea that the V6 would disappear come from?
In my dreams...I see a Turbo 4 LS, V6 LTs, and V8-powered SSs, Z28s, and ZL1s...If the current car was a little more aerodynamic and not as heavy, it might have already had a Turbo 4....But it wasn't designed for this option...
the Alpha platform was.
We need to step away from our need for performance for just long enough to realize the importance of offering a really good inexpensive, economical, attractive, and "fun" (not necessarily fast) base model(s) for the masses.
The only point I think I might argue, 3...is that I think the design is what sells this 5th-generation car...if some compromises (stress "some") in visibility, or convenience are needed to achieve another awesome look...
then that is a worth-while trade-off, imo.