The BMW 520d SE automatic gets 60.1 imperial miles-per-gallon combined, the Audi A6 2.0T multitronic gets 56.5 imperial mpg. According to Autocar's review the estimated fuel economy for the 2.5-liter V6 in the 2012 GS 250 is just 35 imperial mpg. Giving up 20 mpg 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 and putting out 180 horsepower combined. That would put it right in between the BMW and Audi outputs, but 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."