I believe you are correct but it is going to take me a little research to find you the answer to this one. I will do some research for you and see if I can't find that information.
The main CAN bus on all Deere equipment to date is still operating on the 250kB/s. Deere is using the 500kB/s CAN networks for their short bus networks in applications such as Active Command Steering on 7R and 8R tractors. Since the Active Command Steering is completely fly by wire the response time by necessity must be fast. The 500kB/s connects the two controllers for the ACS to the power modules that operate the control valves.
I believe the 500kB/s CAN short bus is also used on S series combines and SPFH equipped with Pro Drive transmissions for the fly by wire hydro control.
I believe that there are a few other applications but in all cases the 500kB/s are used for short bus connections between a controller and a power module.
I cannot find any product that publicly shows the vehicle CAN or Implement CAN as anything other then 250kB/s.
Let me know if you need more info.