I am buying the 328i sedan this week and wanted so...

This is a common issue with most new vehicles, tire wear.  I would check out www.carsurvey.org for reviews on the 2007, and also Car and Driver magazine or their website.  

We dont have the sport pak, so ours is running on the standard 16's. The car has 10 000k on it and we do push it a bit. The tires currently still look factory fresh. So im pretty sure the 17's would still last a fair longer then 15 k.

its not so much that the tires become worn to the point that they don't grip the road by 15k, its more like you start to hear a lot more road noise and the ride becomes a little harsher. frankly, bmw should scrap the whole idea of run flat tires.