IMO, mercedes is sticking with the 3V per cylinder just as a trademark.

but if you compare side by side engines of the same displacement, mercedes' engines have a lower output.

here's a comparison between MB & BMW engines. to note that even though the MB has a higher displacement the BMW still outclasses it. if you noticed, AMG tuned MBs are mostly supercharged to compensate for this.

Mercedes-Benz E240
2.6 SOHC 18V V6
power: 177bhp / 5700
torque: 240Nm / 4500

BMW 525i
2.5 DOHC 24V inline-6
power: 192bhp / 6000
torque: 245Nm / 3500

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SLK 350
3.5 SOHC 18V V6
power: 268bhp / 6000
torque: 230Nm / 2500-4800

BMW M3
3.2 DOHC Double VANOS 24V inline-6
power: 343bhp / 7900
torque: 365Nm / 4900