For Turkey I'm using #5 copper plated in a full choke, however I'd like the option of not worrying about if I need steel shot for duck, etc. Am I understanding that switching to the X-full might be actually a bad idea
Thats the best part of having removable choke tubes. You can change the choke to the best one for your application. It takes 1 minute and can be done in the field if conditions change. You can have an x-full for turkey in the morning and modified for pheasant in the afternoon and full for geese in the evening.
All chokes that come with the 835 are safe with steel or lead. Each tube will shoot steel tighter than lead. That is why they gave you the chart. The modified choke shoots lead to the standard modified choke pattern, but steel patterns to a full pattern with the same choke tube. But you need to pattern your gun on paper to see how each load and shot size/type will actually shoot. My 835 and your 835 may shoot the same load differently.