I have yet to put a K&N on any of my Benz's, but I have one on my off road four by four, and my Vette.
On the '88 Vette with a Mass Air Flow system, anything you do to increase flow is compensated for by the computer. There is a SLIGHT flow increase which may add a couple of horsepower. This is very incremental, you would probably never know the difference.
I use it on the 4 X 4 because it can trap more dust. The one I use on it is 5" tall which you can get under the hood of a '78 Ford pick up.
The K&N is an excellent product for trapping dust offroad or gaining a slight flow increase. If you want to use one for economy reasons, I think it would be false economy. Even if you don't have to buy any more air filters, you have to keep it clean, and to do this you use the kit from K&N which is not cheap.
If you're looking for economy replace standard filters as needed. If you want to spend money for a SLIGHT, maybe unnoticeable performance increase, buy a K&N filter.
'84 Euro 240D, 523K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in