i have same experiences about power, engine becomes fully alive after 4000rpm, below that if you try to accelerate in ordinary trafic with 4th gear, acceleration is nothing special. Now weather here has been cold and humid due rains, and i have been able to activate asr easily when starting gear 1. Things under the hood get very hot even you drive 100 km/h, hot air cannot get easily out from engine compartment. Try air filter assy, intake air tubing,everything gets so hot. No wonder that power is lost in hot climate, because engine cannot draw cool air inside.
I will make a test and remove plastic cover under the engine and try to measure if engine and air intake is cooler. This is not of course cure to base problem, but anyway route to find pittfalls in MB design....Hm, i'm starting to think that this car needs intercooler and supercharger.