FYI:
My car a 1999 996 has the same issue, I have had it in to Porsche dealers multiple times and alignment looks OK on the machine. The tires show inner wear on the rear similar to the above issue. I think the problem is there is not enough adjustment on the rear camber. The rear camber arms or control links do not allow adjustment on stock 996 applications. After market or Performance upgrades can be added that allow these suspension points to become adjustable, as they should have been in the first place by design, my opinion.
Ref:
http://www.******auto.com/ShowItem/253525%20Porsche%20996%20-%20997%20Rear%20Adjustable%20Control%20Arm%20Set.a spx
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