Negative camber (the type you would get from sagging) would not cause wear on the outside edges of the tires, so I would have to question the diagnosis. That being said, how excessive is the negative camber? The printout should have the spec. Have you looked at the E90 M3 BMWs? They probably come with a similar amount of negative camber from the factory. Excessive negative camber alone will not cause uneven tire wear - improper toe will cause that more than anything. Just get the toe fixed, and if at some point the negative camber becomes slightly excessive, don't worry about it. I personally wouldn't spend $$$ on such an issue.

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280