not sure if you have found this or not, but it is also configurable in the options. if you reed the very next line :
GConfig->GetInt(TEXT(“DevOptions.Shaders”), TEXT(“WorkerProcessPriority”), PriorityModifier, GEngineIni);
its reading an option from Engine/Config/BaseEngine.ini
in that file do a search for WorkerProcessPriority which will also equal -1, simply change that and it will use that value instead.
the following numbers match priority:
-2 = Low, -1 = Below Normal, 0 = Normal, 1 = Above Normal, 2 = High