I'm new to Mercs, although I have extensively used K&N and 3A Racing (AAA - Australian Auto Accessories) Pod Filters and panel filters on classic 900 Saabs. Overall my experiences support the advice given by John Jones Jr.
Generally, I did notice that the cars became more responsive with a Pod filter, whereas the panel filters offered a very subtle increase in response. The advantage of using a panel filter is that they work with the existing OEM induction systems, whereas a pod filter requires the existing air filter box to be removed along with possible alterations in the pipework layout. My main motive for using pod filters was to improve the throttle response rather than add more gee gees under the bonnet.
Does the increased responsiveness of a pod filter mean that it has added more horse power? Not necessarily. The engine power could be the same, but you are accessing more of that power. Thus the increased throttle response may give a false impression that the pod filter has 'increased' the hp. It is a bit like people who lose weight claiming that they now have "more energy", when in truth after losing weight they have less weight to carry around, which feels easier than before, thus giving them the feeling that they now have more energy.
The induction noise of the K&N Pod made the cars sound "Boy Racer", hence I switched to the quieter 3A Racing pod as I did not fancy being cop bait!
Mounting the pod filter directly onto the Air Mass Meter made a slight improvement in response over mounting it at the end of a long pipe connecting the pod to the air mass meter. This may be because the air has less distance and less pipework to travel through when mounting directly onto your AMM. Placement of the pod filter is important too. If you mount it in front of the engine, like say where the existing air filter box is/was in the CLK320 W208 behind the driver side headlamp assembly, then cool air hits pod filter. If the pod filter is mounted behind or on the engine then the air going into the pod filter and induction system is heated by the engine, resulting in less dense air so a weaker combustion.
In all honesty, I found the Pod filter to be more noticeable upgrade than the panel filter both in terms of induction noise and throttle response. Therefore if you do want to use a filter upgrade and don't mind removing your air filter box, I would also suggest the pod filter option like the K&N 57i kit. If however you wanted to keep your existing induction system and use a panel filter, then it is harder to justify the cost over the minimalist gains in responsiveness.
Talking with friends and family in the automotive industry and motorsport, the consensus is that to notice any significant increase in hp would require a combination of modifying the engine portings, re-routing the induction flow, remapping the ecu, and upgrading the fuel delivery system in addition to using a pod filter. So unless you are racing or a die hard petrol head, is it worth all the extra outlay? At the end of the day would any of us notice if the power output had increased by a meagre 7 hp from 218 hp to 225 hhp during our everyday drive?