Correct

The GP39 has 15% more horsepower, so compared to a GP38 it will either pull about 15% more train or the same train about 15% faster. But at 10 mph, both would pull about the same.

The GP39 has 12 cylinders and a turbocharger; the GP38 has 16 cylinders and two roots blowers - some railroads had a preference as to which was more economical to maintain.

In my experience, turbocharged EMDs do not like constant low throttle operation (wear on the turbo clutches). It has been common for railroads to prefer assigning non-turbo EMDs to yard or low throttle service. But you can find a GP40-2 or GP60 in low throttle service because that is what is now available.

In higher altitudes, the turbocharged GP39 would not suffer the same horsepower output loss compared to a GP38.

EMD made the GP15 and MP15 in both the 12-645 with roots blowers and a turbocharged 8-645 (GP15T/MP15T) due to customer requests. This compares to a F7/GP7 with used a 16-567 to get the same 1500 horsepower. The repowered locos and new EMD Canadian Pacific GP20ECO units use the turbocharged 8-710 for 2000 hp. I'm not aware of EMD offering the 710 engine with roots blowers.