So far? Upgraded rims/tires. The wider setup and more performance-oriented tires add a wonderful amount of grip to the car without adding too much drag (maybe 0.5mpg). The Nurbies are also much lighter than OEM (I think 9 - 12lbs each), so that probably gains me back that 0.5mpgs.
Combine that with the aesthetic upgrade they give and I'm sold!
Have to say, the Bilstein B8s are pretty great so far too. Sounds like my rears are starting to show their age as well, so I might be replacing them soon too - definitely going B8s all around.
Thanks! I went slightly wider on mine and my TPMS sensor keeps tripping...the tire guys can't figure it out either. I've been searching for B8s and can't find the rears unless I'm looking wrong. Did you pair it with any lowering springs right now or is it still on the stock springs?