Bulletstorm is going to earn a 'Mature' rating no matter which region it releases in -- this much is a given. Even so, German versions will come heavily censored, even with the game's 18+ rating.
The German Youth Protection Board (BPjM) has banned graphic/violent games in the past, even developing a reputation for its quick actions against offensive media; but games already given a USK seal -- meaning they've been rated -- are off limits, as is the case with Bulletstorm. That said, the BPjM can still censor rated games, so like many other 'Mature' titles before it, Bulletstorm will be appropriately butchered before hitting German retail.
EA faced a similar issue with Medal of Honor, which wound up on the BPjM's ban list for containing content so offensive it could have been prosecuted criminally. Activision's Call of Duty: Black Ops, on the other hand, got by with heavy censorship.
That hasn't prevented German gamers from simply ordering uncensored versions. Retailers like Amazon.de, for instance, revealed that PEGI rated games tend to sell better than German versions.