I just installed the K&N 9" cone filter. This isn't the kit made specifically for the CTS but accomplished the same thing and is a LOT CHEAPER. All you need is the filter which you can order for 40-50 dollars. It's part number RE-0920. The filter is a standard 9" long cone filter with a 3.5" flange. If you disassemble the stock air-box, you'll find that the inlet inside the box attached to the MAF sensor is exactly 3.5". Everything fits perfectly! I pushed the filter onto the inlet and tightened the metal collar around the flange. Their seems to be enough support from the half of the airbox to keep it from flopping around. I'm going to see if I can cut the bolt anchors from the unused half of the airbox so I can use those to give further support this weekend. After installationg, the first thing I noticed was the much more throatier sound. Under light to normal throttle, there is no noticeable difference in performance or sound but once you tap the throttle a little more, a very throaty engine starts to roar. I can feel a little performance gain (maybe 7-10 HP) using my patented Butt-O-Meter horsepower measurement device, more pressure on my butt from the seat when exactly = more horsepower. I can now perform a burnout without dropping the clutch or fanning the clutch. I wish I could post pictures but I have no digital camera for now. Once I get one I'll be sure to post them up in this thread.