There are no surprises here. With a curb weight of 4,000 pounds or more, and V-6 engines, the 2014 Cadillac XTS gets about the mileage you might expect of a very large sedan.
The EPA pegs the fuel-economy numbers at 17 mpg city, 28 highway for front drivers or 17/26 with AWD. That's not bad considering the XTS' size and luxury, and capacity to carry five for long periods of times.
As for the new Twin-Turbo V-6, there are no results yet, but expect mileage to be about the same—perhaps down no more than 1 mpg.