There is a guy on the Roadfly board that made his own twin turbon system for the 850 and is making 444 HP and 524 TQ.
http://bimmer.roadfly.org/bmw/forums/e31/5537028-1.html http://bimmer.roadfly.org/bmw/forums/e31/5537988-1.html
And on this next article is one of many stories that involve a high mileage V-12 that was abused. Surprisingly the only thing that was wrong with it was that the cams were a bit worn after 270K kilometres. So that is why I would not trust the dealership where that guy had his engine replaced.
Now if you are confused and the car was an 840, that is a whole different story. The 840's had a problem with the Nickasil engines. They had weak/bad cylinder walls that were easily scored by the piston rings.
http://bmwe32.student.utwente.nl/sea...0rebuild_1.htm