GR10 and Haute Route Pyreness in France

I am planning an expedition run of the GR10 in France next summer. This goes from the Atlantic to the Mediterranean through the Pyrenees mountain range (approx 850km). Has anyone hiked this before? If so, what are your opinions of it's runnability? I am also trying to get info on the Sentier Piemont – does this connect or replace a portion of the GR10?

Thanks,
William