Mine is an '07 X500 and she definitely runs better on premium. I tried regular at first, then switched and noticed the difference. Engine is smoother from idle to hiway speed. Acceleration is about the same, or ever so slightly better. Mileage is better from about 5mpg.
Overall, my Puff, named after the magic dragon, runs better, smoother, and happier on premium. Haven't tried mid grade yet. That's U.S. premium. The extra few cents are worth it to make Puff happy doing what she loves to do most, roll down the hiways and byways.