There is a lot of food for thought on this subject in Francis Chaboussou’s book La Santé des Plantes, chapter 7. I believe the book was translated into English.

What I understand, simply put, is that the grafted vines have mediocre protein synthesis. If that is so, then perhaps sap analysis and nutritional corrections can help?

If it does and photosynthesis becomes more efficient, would the exudates from the rootstock start building a more adapted microbiome?