I have 2010 X5 35d, I can confirm that the the side vent flow is very very less compare to the middle ones. I had been to the dealer once intially at the begining but the answer was "It is supposed to be working the same way as in the other BMW model - Not an issue"

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Turbo-Diesel trucker
2010 X5d – Vermillion Red/Saddle Brown|ZCW|386|609|620|6FL|6UH
Ordered 10/20/09 – Production # 10/23/09 – Production Start Date 11/23/09 - Production Completion 12/03/09 - Dealer Received: 12/07/09 - Actual Delivery 12/12/09
2007 Acura MDX (Tech and Ent) – Nimbus Gray Metallic/Black