The designer is responsible for the proper specification of the piping, and using recommendations from the internet might be considered a high risk proposition.
Having said that, the design temperature is above the max permitted for carbon steel, and low alloys such as P11 are marginally acceptable if corrosion and erosion were not an issue. Except that they are an issue in this case, depending on what the slurry contains.
The design conditions you listed suggest a supercritical oxidation process is being planned. I think you will need to focus on corrosion, espescially if the water in the slurry has remnant oxygen in it. It is remotely possible that the corrosion could be worsened by surface reactions with the oxygen and the iron scale on the pipe walls; something unusual can happen at these pressures and temperatures.
"Whom the gods would destroy, they first make mad "