[tasdrouille]

On a sedan, you have to trace a tangent between the back of the roof and the rearmost part of the trunklid (including any kind of spoiler). If the angle of the imaginary line is in the vicinity of 13 degrees you have a winner. If the angle without spoiler/ducktail/extension is already 15 degrees or less, any appendage is probably superfluous. I do not believe anything significantly pointing up past 10 degrees could do any good for drag, except maybe just a little sharp "tripwire" bump (as they did on the volt) for the flow to leave cleanly as it tends to stick (there's a small section of reverse flow) a bit on rounded corners.

Big dave, my guess is that if the trunklid is long enough the flow will to reattach to it anyway, so that ducktail would not serve any purpose. On a short trunklid for which the angle is greater than 15 degrees and the flow would never reattach anyway, then it would be benifical (as for a hatchback, the volvo c30 with the downward pointing ducktail comes to mind).

Most drag reducing aero attachments are bandaids for poor design or stylistic constraints, when they even work.

If I remember right, Hucho reduced the Cd of the vw sirocco by 0.01 with a well designed lip spoiler.