I have a 2007 325i with the 2nd set of Bridgestone Turanza RFT (16"). When the 2nd set had about 28000 miles (6,ooo miles ago or so) I had them siped at the local Discount Tire. I was surprised not only how much better snow/ice traction they had (this was done early this year during a Michigan winter), but also the tires are significantly quieter on the dry pavement. According to the company that manufacturers the siping machine, one of the benefits of siping is a quieter ride as it essentially makes the tire more compliant, flexible, and this will also improve your treadwear.

For the nominal charge ($10 each tire I believe), its worth it to have your RFT's siped, especially since the tire options for RFT's is somewhat limited and the cars obviously don't have spares (or room for them without sacrificing trunk storate) if you switch to non RFT's.