I have heard 3% from BMW, but I don't know if that was for specific models or all awd variants.

I have heard of other X3 and X5 owners having problems with 2% calculated difference, with constant warning lights as the system fought to keep wheel speed constant front/rear. With my own vehicle, I would go to 2% maximum. If I had any warning lights I would change out the rest of the tires, or get the new ones shaved down to a similar tread depth.

If by 'above spec' the engineer you heard from meant that one tire at minimum tread depth plus very little, compared to the other tires being new, would not result in potential problems, then I wouldn't believe him.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue