Have had some running issues in the past that we thought were due to oil contamination, but I was using an oiled Simota. And MAF cleaning is as easy as spritzing it with a bottle of Elektra contact cleaner and letting it drip dry. Have had absolutely no issues with K&N, even after re-oiling the filter.

You can sometimes find residue on the inside of the intake pipe, though.

Consensus is... if you clean properly and don't over-oil your K&N, you should be fine.