A similar answer to your follow up question. In Germany cars are taxed based on engine size. As a result the German car makers number one design goal for engines is to get as much power from as little displacement as possible. As a result, these engines are at their maximum state of streetable tune.

An engine is a pump. When the pump can flow more fuel/air in and exhaust out, it makes more power. Your M103 flows at least as good as any two valve streetable engine in the world of it's size.

There's just not much room for improvement unlike an American engine of the same vintage.