Custom F0 map is often not needed. But for HD result at a close range, it works nicely.
And yes, there is a difference between Substance and Sketchfab, but for the better : In fact, I only enter F0 values in a custom linear map inside Substance Painter, so obviously it doesn't appear in the viewport when working with the Metalness Workflow. But once I put it in Sketchfab, it refines the quality without breaking the initial render. So everything is fine between Substance and Sketchfab.
To be honest, this model was a test for many things, including custom F0 of course, but it is certainly not the most obvious showcase for an F0 map. I think that a model combining cristals, clothes, skin and liquids on a single texture will be a much better example of F0's impact on the final render. I will think about it for my next model.