Racism is certainly deplorable, but two of Sarrazin’s quoted statements are plausible and seem to be confirmed by historical experience. :
1. “Muslims immigrants don't integrate as well … not based on their ethnicity, but … [because of] their culture of Islam.”
I am fairly knowledgeable about Islam and its European diaspora, and find this a plausible statement. It's controversial, but there’s a lot of evidence for this view. Perhaps you have not noticed, but Islam is fairly elitist and accordingly tends to encourage the formation of foreign bodies encrusted in non-Moslem countries. One of the main reasons Turks don't date Germanic Germans or intermarry with them is the prudish sexual attitudes encouraged by Islam. They’re not slutty enough to be Germans. Tant pis pour eux.
2. “… influx of immigrants from the Middle East which would overwhelm the indigenous population … [as in Kosovo]”
This view was espoused by Samuel Huntingdon in his Clash of Civilizations. He provides considerable statistical evidence and case studies to back it up. Huntingdon hypothesizes that several genocides against Moslems were a panic response by prior inhabitants faced with the threat of being outbred and outnumbered on their native soil. Of course I do not wish to justify the crimes of the Serbs against Moslems. The Serbians f__ked up big time in Kosovo and Bosnia, because they’re unsophisticated boors and they stupidly followed the classical Fascist dictator Milosevic on the road to perdition. But the claim that the Serbs were outbred in Kosovo and Bosnia seems indisputable.
Consequently these two claims are not “wacko” statements in any sense of the term. So they should not be dismissed out of hand, but instead discussed, followed up and investigated.
Sarrazin’s skill in financial mathematics does not give him any standing at all to make ethnically biased remarks. Math and political science require completely different skills and totally different approaches.