I just fitted the filter, no other mods. The filter is not listed for the 166 on the K + N parts book, but it is the same filter as the 2.5 v6 156 > 98 models.
The main change as said is the performance a slight increase in induction noise is present. As to the comment as to the extra air box , it seems a good explaination. I think i will try removing it and replace it with a straight piece of pipe left over from my thema turbo.
I bought the filter from Demon Tweeks who have a world wide sales network. But i would have thought there are many other outlets for them.