With the introduction of the turbocharged Mercedes Engine line up beginning with the v12, v8, and v6, it was realized that there were more impactful gains to be made from simple ECU tuning than in the past.
The levels of ECU tuning were no longer 10-30HP and quicker response, but 80-100 more HP. This new level of power can of course reveal weaknesses in the drivetrain, up until the point where it is clear that the turbos are a bit too small... That is of course another topic. I will also write about some of the common issues that are encountered with ECU tuning.
Some of the popular options for ECU tuning are to buy a salvage ECU and have it cloned and tuned, to purchase a hand held tuner, or to have the car brought to a dyno and custom tuned based on the data from the pull.
Some of the popular providers are Renntech, Weistec, OE Tuning, Eurocharged, and AMS. As of 2020 the most aggressive 93 octane M278 tunes will yield approximately 585-hp.
As another data point an e85 tunes with Down Pipes in a M157 will in practice put down 684 HP to the wheels.