If you are a true audiophile, you'll hate the premium system too. I did not get the premium system with my X, but instead upgraded the system with audiophile quality speakers, amplifier, subs and a sound processor. (Get custom boxes fabricated for your subs and hide them behind the carpeted wall in the trunk and you'll not need boom boxes. You might need to put a grille in front of the sub for the free air movement)

As far as number of speakers are concerned, in a 'small' cabin like that of X5, whether you have 10 or 16 speakers, it really doesn't matter. A nice 10 speaker system can sound hundred times better than a stock 16 speaker system if the speakers and amplifier are of good quality and have been tuned really well. Tuning makes hell lot of difference!

Trust me, my system can blow away any 'stock premium system' by leaps and bounds! A friend of mine has Bowers & Wilkins premium system in his Jaguar XF and he feels my system sounds better! I did end up spending lot more than a premium system, but at least I had the flexibility to choose the components and tune the system to my taste.