If the engine is running properly, you should be at max boost by around 2000rpm. If not, most likely the IP is low on fuel delivery off idle, which you can try fixing with an ALDA adjustment. Anyway, 15 seconds 0-60 on a 300SD is factory spec,
AFAIK, and if you're getting those times the car is running fine. If it's more like 20 seconds or worse, something is wrong. It's not supposed to be a 'fast' car, ya know!
:p