At 518RWHP if that engine is indeed making 580BHP at the crank then that would give us an 11% drive line loss.
The reason that the V6 Mustang and V6 Camaro can get the fuel economy numbers that they get is due to such low drive line losses. Consider that the V-6 Mustang is rated at 305BHP and gets a best 31MPG and the Camaro V-6 at 323BHP gets a best 30MPG. Consider that the Honda Accord V6 Coupe wwith 271BHP gets a best 29MPG highway, thats all 3 with automatic transmissions and consider that the Honda is 3,400 pounds compared to 3,450 pounds for the Mustang and 3,750 pounds for the Camaro. When you equip all 3 vehicles with manual transmissions the Honda does even worse as it has a rating of 26MPH highway. Compare that to 29MPG for the Mustang and 28MPG for the Camaro.
So what I am saying is the reduction in drive line loss has given the RWD platform new life as they are now capable of achieving fuel economy numbers that are competitive to their FWD counter parts. The fact that RWD vehicles use to have a driveline loss in the 20-25% range and now are in the 10-15% range is impressive. That some models in the past had a 25% drive line loss and now have a 10% drive line loss doesn't just mean better fuel economy. This also means better performance all around, consider that if it takes 300RWHP to get a 3,000 pound vehicle to 60MPH in 4.5 seconds. At a 25% drive line loss it takes 400BHP to achieve 300RWHP, this means a engine that not only consumes more fuel but a engine that is bigger and heavier all things equal. Also the transmission has to be beefier to handle 400BHP going into it, so the the drive line gains weight. So in order to keep the 3,000 pound vehicle weight you need to use lighter (more expensive) materials. At 10% drive line loss you only need 375BHP to do the same thing, though the difference of only 25BHP is a small difference. This difference becomes more pronounced as the power levels go up. When you are looking at 700BHP range at 25% loss you get 525RWHP and at 10% loss you get 630RWHP a difference of more then 100RWHP.