Whenever you change to a camshaft with increased duration, you are basically shifting the power band to a higher RPM range. Additionally, one of the most important aspects of building power, is for everything to work together. That is, the cam timing, intake system, ignition timing etc., should be well matched.
Although I don't doubt that the camshaft would give you an extra 15 horsepower, it will be at a higher RPM range for which the other components were not designed. And it will be at the cost of what little bottom end torque the engine has.
In Germany cars are taxed(heavily) based on engine displacement. For this reason, the engines are already designed to make quite a bit of horsepower for their size. This is done by building that horsepower at a higher engine speed range. In an American V8, there is a lot of room to get more power by shifting the power band up, with a camshaft. The German engines don't give you much room for this.
'84 Euro 240D, 516K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in