The BMW 520d SE automatic gets 60.1 imperial miles per gallon combined, and the Audi A6 2.0T Multitronic gets 56.5 imperial mpg. According to Autocar, the estimated fuel economy for the 2.5-liter V6 in the 2012 GS 250 is just 35 imperial mpg. Giving up 20 mpg, even of those smaller English gallons, is a huge disadvantage.
But Lexus doesn't do diesels, so it's developing what is thought to be a four-cylinder lump hooked up to an electric motor, putting out 180 horsepower combined. That would put it right in between the BMW and Audi outputs, but fellow UK publication Auto Express suggests a 50 mpg rating, perhaps due to an almost-certain weight penalty. The exact details on the hybrid setup should come "imminently."