Hi Colin,
Have done a few short to medium runs with the K & N performance filter and I humbly concur with your comment about the additional noise under hard acceleration. This was rather more than I had anticipated so I would certainly advise anyone considering these filters to be aware of this since it's not to everyone's taste of course.
However, although I was a little surprised by it, I do actually like the more powerful engine sound it provides compared to the slightly harsh revvy sound that it makes normally with the standard filter under hard acceleration - so it is down to personal taste and in normal driving it is not noticeable so to me the best of both worlds.
I calculate that fuel consumption has decreased ever so slightly (despite using more hard acceleration to assess the increased sound and any power improvement and definitely not a "placebo effect" as you suggested) - I'd calculate about 1 mpg better on short runs and nearer 2-3 on a longer run, plus a very slight improvement in acceleration, but most noticeable was an improved throttle response instead of the slight hesitation and delay that usually occurs throughout the rev range.
So all in all it was what I had hoped for and anticipated, with the exception of the aforementioned increased sound under hard acceleration which I do actually prefer.
I will let you know if I do get any issues with the "MAF sensor getting coated with oil" as you put it - but again I find it hard to imagine that these filters are designed to cause such an issue and decrease engine efficiency - but I'll keep an open mind nonetheless and hope you're proved wrong about that aspect.
It would be interesting to hear from anyone else who fits one of these in the meantime and/ or has experiences with either positive, negative or neutral results.
Tony