The European octane rating is RON, not pump octane or anti-knock index (AKI) as used in North America. Figure about 5-6 points difference in the two measures, although it isn't a direct conversion. That means the 99 you saw in Europe is for all intents the same as 93 in North America.

Putting in a fuel with a higher octane rating, or anti-knock index, does absolutely nothing on a vehicle that can't take advantage of it. The X5 can't benefit from that extra knock resistance. A heavily modified engine (different pistons, higher compression ratio, etc) or one using forced induction, can often take advantage of a higher anti-knock rating. All others are simply throwing money away, unless the higher octane rating is cheaper It doesn't go faster, develop more power, get better mileage, or thank you for the treat. It can be harder to start in cold weather.

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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