I think there has been some degree of misconception about the so-called F-duct. First, I would like to clarify why it is called an F-duct. The original duct happened to emerge from the bodywork coincident to where the "F" of Vodafone is displayed on the nose. Funny stuff, eh? But true.
Ok, but the more obvious misunderstanding is that the duct is intended to increase straightline speed. Most of the comments from the media, the bloggers, and from the forum posters seem to indicate that the duct is for lowering drag to increase the speed on the long, straight portions of the circuit. But, I think that is a misconception.
The intent is to reduce the drag on the straights that results from running high downforce wings in the high speed corners. The idea is that the perfect downforce setup for high speed corners is prohibitive on the straights. The optimal setup for the corners causes reductions in top speed straight sections and make the cars vunerable to overtaking in the braking zones.
If working properly, the F-duct doesn't result in higher staight speeds, but instead in higher cornering speeds. The idea is to dial in high downforce for the high speed corners and then to mitigate the drag on the straights via the F-duct. So, you match the following cars straightline speed, and then you enter the corner faster because of more agressive aero.
There is little advantage to straightline speed if one must brake down to slow cornering speeds. If the F-duct works properly then one's staighline speeds match the others, but one's cornering speeds surpases the others.
The formula is this: high downforce for high cornering speeds & F-ducting to reduce the drag caused by the high cornering aero setup on the straights.
It does increase the straightline speed as compared to when you do not employ the duct, but that is a mitigating function.