I tried the modification today again.
I disconnected the electrical connector on the upper right side of the engine which is supposed to control the flaps via vacuum tubes. There is a similar connector on the lower right side, but it is for the valve in the air box.

I turned the ignition key and the engine came to life with the usual flat six growl. I stepped out of the car and checked that there was pressure behind all four exhaust tips. I also checked that the exhaust button in the center console did not have any impact on the sound. Conclusion, the modification is in effect and the flaps are fully open.
I lowered the windows and drove off. The sound was louder, especially when the RPM reached 4000 and the up to the redline. There were hints at pops in the exhaust when shifting down. More defined than before. So clearly the modification works, but... it is not the same big difference as it was on the 997.1 PSE. The really loud pops on the overrun are not there, it is more a rumbling and muted sound.
I suspect this is all due to the infamous center silencer which restricts the sound before it hits the main silencers on the left and right side. Speculation, replace the center silencer with straight pipes (a.k.a. x-pipe) and the 997.2 PSE should sound like the 997.1 PSE. Together with the modification or Gert's device the exhaust sound should then be real nice!
Check this video for comparison http://www.youtube.com/watch?v=AUTmnOvuW6I
The consensus on other forums, where owners have installed the x-pipe, is that the center silencer is the culprit for the more muted and high pitched sound from the 997 DFI engines.
The center silencer also appears to be highly restrictive in the air flow. Shark Werks did a dyno run with and without the center silencer and the difference was 12 HP 
This is how it looks inside:

The replacement pipes are straight leading the exhaust from one side to the other, rather the the stock U-shape which leads the exhaust back to the same side. It is this U-shape which is supposed to be restrictive.
If all this is true, what is the purpose of the center silencer? To make the DFI sound more muted and high pitched? To restrict the power?
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997.2 Carrera S in Carrara White. PASM-Sport Suspension (-20 mm), PSE.
987.1 Boxster S in Arctic Silver. OZ Racing Ultraleggera Wheels, H&R Monotube Coil-Over Suspension, H&R Anti-Roll Bars, Sachs Racing Clutch, Single-Mass Flywheel, IPD Plenum, GT3 Throttle Body, Recaro Pole Position Seats, PSE, Alpine Head Unit and Amplifier, Focal Speakers