On my 1980 300SD the linkage from the accelerator pedal to the injection pump was mis-adjusted. I had poor acceleration until the turbo kicked in, then decent acceleration. After adjusting the linkage to get more movement on the IP, it accelerates good off the line.
The procedure I used is to hold the accelerator pedal down, just before pressing the kickdown switch (underneath the pedal). Then adjust the linkage so that this position gives full travel at the IP. It's been working great ever since.