Good article regarding the AFM: http://frwilk.com/944dme/afm.htm
Ignore his comments on not adjusting it.
No need for the aluminum tube, just buy a new
rubber intake boot and stretch it over the .5" larger AFM flange. To do this soak the boot in boiling hot water for a few minutes then pull it over. It'll fit.
There are better than K&N (like ITG) but the issue with the M20 engine bay on cars with cruise control is a complete lack of space. Most 3" filters will not fit in there, only K&N has a diverse enough catalogue to provide a proper sized M20 cone.
Here's a clip of my car at an auto-x. Only mods are a MarkD 91 chip and the K&N cone: http://www.utsc.utoronto.ca/~04chang...0Aug%2004.mpeg