According to the research I've done on the ECU tunes, they increase horsepower and torque, turn off the exhaust gas recirculation valve (EGR), all while improving fuel economy if you keep your right foot out of it.

Mercedes used EGR to help meet government imposed emission standards by routing the exhaust gasses back through the intake process to be re-burned. The problem is the soot associated with diesel exhaust gasses gets built up in your intake system over time and reduces performance, eventually even plugging up the intake system in worse cases. Also, that soot contains abrasive particles. No doubt those particles don't do anything good when they enter your cylinders. Stopping EGR should have the effect of prolonging engine life, at least in my opinion.