Originally Posted by lo_jack

A boosted engine is going to need more octane (oxygen, or just fuel in general) to keep pre ignition/ detonation at bay, as the compression ratio is going to be higher and the cylinder head temps higher. The computer will try and correct this by retarding the timing, btw.

That is a given, but doesn't effect the turbo per se. That is the result of wedging more and hotter air into the chamber. At best, if you ran more octane, you could perhaps increase the boost slightly before you got detonation again. Or spray methanol, or run an intercooler.

X5s already require premium because of the heads and pistons (the comp ratio has to be 10.5 or more to 1), but that doesn't mean adding more octane beyond spec will do anything better, unless you have a problem, like excessive deposits that would appreciably change the size of the combustion chamber (hence the Seafoaming).

Beyond the point that the engine requires more octane/oxygen (you could just dump more fuel by keeping the injectors open longer; fuel cools the intake charge) adding octane does not do anything.