According Brawn (AMuS), the change also brings better engine cooling, in addition to a better flow to the rear wing and reduced drag.
AMuS says that the aero updates alone should bring 0,3-0,4 seconds gain, without taking into account the laptime improvement the extended wheelbase should bring.
Well, you specifically said that it's not about reducing the drag at the airbox. So in that sense, I'd say yes, it's disagree
I seriously doubt they'll ban the F-duct, but as for the DDD it's true. And it was the same for KERS, and for the tyres now. You can't cut costs, be green, AND solve overtaking all at the same time.