The big issue with Merc spark plugs is the fact that they are non-resistor. The responses don't address this issue, but if you add additional resistance to the circuit, it is going to consume some ignition energy (I-squared-R), so the best bet is to use the non-resistor plugs recommended by Mercedes.
Another thing to consider is that the spark plug is "cool" relative to the 4500 degree F flame kernel that is initiated by the spark. All those ground electrodes can act as a heat sink and quench an insipient flame. "High Performance" spark plugs use very fine center and ground electrodes, which minimize the heat sink.
Multiple ground electrode spark plugs are just marketing hype. I know of NO manufacturers who install them as original equipment, and they are looking for every last bit of efficiency as many are on the ragged edge of not making their CAFE numbers and paying big fines. If multiple ground electrode spark plugs had any value, OEMs would install them as original equipment. But they don't. There's a message there!