I've run the Wunderlich Blue Air Filters for 4,000 km since December 2005.
The filters do give slight performance gains. But compared to the awesome sound and power the bike had before, the filters' gains are difficult to detect. You'll find the engine doesn't appreciably sound louder, feel more powerful or rev quicker.
One performance modification that you will feel is replacing the pipes. I definitely feel the difference that the Remus full Titanium header/cat.eliminator./silencer system gives the bike. The Remus pipes definitely give the bike an awesome sound, shed weight, and increase the power.