The 3800 (series I was the engine out of the Buick GN and GNX) is a great pushrod V-6 and is still used in a FI version on the Pontiac Grand Prix GT (formerly GTP from 1997-2005. From 97-03 it was the Series II engine w/ a Gen III M90 Eaton Roots style blower while 2004+ had the Series III 3800 that had a Gen V M90 and drive-by-wire throttle body amoung other improvements), Buick Regal GSX, Chevy Monte Carlo SS (with the 3.8L SC option), Pontiac Bonneville SSEI, and Buick LeSabre.
An example of how far you can go with that engine:
(that's what I'm dropping into my Grand Prix this week.)
However the blower configuration there is not applicable to our vehicles. That, very much, is a FWD only application. I think the Aura's VVT V-6 is also set up for a FWD application. Much harder to put a FWD app. engine into a RWD platform. However, GM has the ability and, if they build on the technology that we see in the 2.0 like direct injection, you'll see these new V-6s make even more power.
As far as fuel ecconomy, that beast of an enigne above gets about 30-32 mpg highway. City sucks azz but cruising it's just great! I could definately see a FI V-6 in our cars...if Mallett can fit an LS2 with a Cent. SuperCharger into these cars...