What are peoples thoughts on these? When I changed the oil and filter on my 3.6tdv8, I thought I'll change the air filter aswell only to realise there was a performance filter in the housing albeit very dirty. After replacing with the standard paper element the engine runs smoother and quieter, not a big fan of them myself, wondered what others thought.

I guess on a standard car they aren't really worth having but if you FF is mapped I'd say defo a good thing , getting more air through than a paper equivlant , the free flow design of them does increase noise levels a tad due to increased airflow , I was looking at one for my td6 this morning as it happens

Pretty much all diesels run with excess air. It would need to be highly modified to get any improvement.
Also some think that over oiling them can lead to maf sensor issues. Go for a dry foam one if you do go for it.

I had K&N on my for 12~13 K km and felt no difference so went back to dry filters.

I have heard stories of peoples experiences when using K&N filters and maybe caused by being a bit heavy handed when oiling them.This has resulted in oil vapour being sucked through the MAF, coating the sensor and causing air/fuel mixture errors.
I did use K&N ones on my classic RR (new 3.9 dropped in with stromberg carbs re-jetted).
To be honest didnt notice any difference in performance just a louder sucking sound when giving it the beans.
Personally I would stick with a paper filter.

Pipercross for me, sadly no change Many thanks
Paul