The MAF problem seems do depend on the design
I personally know 5 E39 540i and M5's ( they use the same model MAF) that had to have their MAF replaced because the engines started running at half power due to contamination from K&N Filters after a few thousand miles. You can read up on the problem on www.m5board.com
where there are hundreds of such cases. You have to replace it, cleaning doesnt work and a new one costs $$$$$$.
But then on the E46 you never hear of such problems, so its related to the MAF design
Now who is going to go and test the E90's MAF to see if it likes K&N type filters or not ???