If the FIA were to make DRS available all the time during the race BUT decrease the normal rev limit to 16,000 rpm, and only allow the full 18,000rpm to be available when a car is within 1 second of the car infront (i.e. to aid overtaking) I reckon the car's lap time would be the same, but since they would have less power available they'd burn a bit less fuel...
The technology is already there, its just the software needs a little bit of code to be re-written to make it happen....
The graph below shows two simulated laps of a typical F1 circuit... one lap with DRS available at all points but limited to 16,000rpm, the other not having DRS available but having 18,000rpm (i.e. a "normal" race lap with this year's cars), notice how both laps end at 91 seconds.