What's the problem with using UP for the Tartine recipe?
I have done that many times successfully.
You can manipulate any flour for any application, and you can get great results. But for Tartine, which is really a french loaf at heart, an 11-12% winter wheat flour yields that best results. That is the intended application for that kind of flour. Like I said before, all protein is not created equally. Spring wheat flour, such as UP High and Ultimate Performer, in general, is high in protein but not as high in protein quality
. Try your Tartine loaf with Giustos Artisan and see if you like the difference.