Folks,
I just installed a K&N performance filter kit on to my 4/4 (2009). It was a bit tricky as the kit was intended for use with donor parts from a Ford Focus but they are not the same as the parts removed from the Sigma. However, it went on directly. There was an extension hose for the crank case breather to connect into the side of the air filter but the cold air hose that came with it was too short to route to the front of the car so I routed it as described in the (3) photograph here.
http://s448.photobucket.com/uploadPerformance - Not much different to the K&N filter i the standard air box and a ram air system (cheaper!). It certainly seems louder and I probably need a good run to get a feel for performance properly.
Over the weekend, I received a beautiful from valance from Cain. I will source cold air tubing to route from this to the new filter arrangement.
With regards
G