Aerohead:
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Smoke flow images of the 911 would really help.
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I have posted such images in the Random Wind Tunnel Images thread.
What I recall is that on the coupe detachment happens at the top of the rear window where there is a 3/4" step down and a slot vent for the cabin.
On the Targa I have found no such image and assume a similar detachment at the trailing edge of the targa bar that has about 1/2" step down at rear window.
In the many searches I have done over the years I have found no documentation to support my personal hypothesis that the powerful cooling fan drawing air down through the rear deck lid alters the airflow and pulls the entire air flow over the car down sooner.
If I recall correctly threads in Pelican Parts revolving over substituting an electric fan for the belt driven one, a 35 hp motor equal to that powering a Prius would be required. Such a power demand could not be supplied by a alternator/generator and would require the weight of batteries.
To put the power of a 35 HP fan into perspective, my 11 foot long 6 foot wide hovercraft is 35 HP, it can carry about 400 lbs and go about 45 mph over still water, 65 mph over flat ice.
The fan on the hovercraft is 30 inch diameter or about 2.5 times the size of the Porsche cooling fan so it does push/pull a lot more air.
I cannot offer up data to compare the two, just suggesting that to ignore the draw power of a 911 cooling fan is to ignore a powerful physical force.