He's 25 years of age, only had one good season in France and has done ok in his first season in Germany.
12 goals in 34 caps for Gabon.
You also state he played as a winger at Dortmund so feel free to explain how he is a proven striker?
A striker is someone who score goals and scoring 16 goals in a new team in his first season by not even playing as a main striker is a good proof to me. He also scored 18 goals in 2011-2012 season. So in brief, he scored 18, 21 and 16 goals over the last 3 seasons, is that a good enough explanation?