Well, as newbie I have no Mercedes perfromance knowledge.
However on MR2 Turbos the K&N filter (FIPK not just the drop in unit) gained 15-20 hp on lightly modified cars we tested. As far as the cleaning ability? We found that Foam filters cleaned better, but any filter that uses oil is far better than a "dry type" or paper. Turbo cars show this very easily by the amount of grit on the intake side of the turbine.The K&N out cleaned and out flowed the stock airbox system hands down. removing the airbox in the MR2 turbo also netted a 17 degree lower temprature in the engine compartment. I think on Mercedes in particular the gains are not as large so members are less likely to recomend them. go to any other message board, the people there are cotton element filter crazy wether its a K&N or not. But I do believe that any engine has something to gain with a higher flowing intake.A Mercedes just doesnt have as much!