For the 335d:
it is an effective trade-off of quicker spool-up and low and torque vs. high end performance. In single turbo setup the turbo sizing is very sensitive topic. Put a small turbo in it and U'll spo9ol up quicker and pull from lower rpm, but U'll loose your breath at higher rpm's. Vice versa with turbos too big - U'll be able to compete gasoline engines, but until U get there... ouch. So the BMW sd setup should cure both of those handicaps - 286 HP and absolutely insane torque, BUT - it's not a sports car engine and the 335d is not a sportscar, even if it wears the E92 suit.
On the other hand using such serial setup on gasoline turboed engines would be effective, too, but would be prone to failures due to higher exhaust temps of gasoline engines or I dunno why. Maybe someone wiser could tell us?