I think you are correct on the overloading, I can only get 3200 RPM at WOT and I also noticed my fuel consumption at any given RPM ( except WOT) is much higher than what is shown on the Cummins chart. I now have 450 HRS and the turbos always had a little discoloration but last week they look like they were really over heated.
For this poker run, I ran the engines at 80-85% load , 50-55MPH where normally I cruise at 45MPH 70% load.