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I still think tires is the biggest difference. Porsche uses the exact same bodies across a lot of models. Between the Cayman S and non-S for example, the drag is the same, but the S is over an inch lower. Lowering 1 inch takes away as much drag as adding 1 inch of tire (mainly in the back). I think the number cited for each of these changes is about 0.01 Cd, which is quite a lot (3% of the whole car's drag).
Same story with the 911, you can see the 4S models with their fat tires having a couple hundredths of Cd added, but it's only an inch or so of tire width making that difference.
For the MB CLA, we're talking a huge change in tire width from the BlueEfficiency up to the AMG, I could see that alone explaining half of the increase in drag.
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