We did a study on the use of semolina flour in pizza dough a number of years ago. We found that for all practical purposes, 25% substitution was the maximum we could go before the finished crust became excessively tough and chewy. We thought 15% was a good working level. Because the semolina flour has a much larger particle size it is slower to hydrate than regular flour, so care must be taken to ensure the dough is properly hydrated. At first the dough will appear to be wet and sticky, but just like with a whole-grain dough, it will improve as the semolina hydrates. We did not detect an appreciable change in flavor when the semolina flour was used. As to using semolina flour either in part, or in total as a peel dust, because of its slow hydration it works great. I use a blend of equal parts regular flour, semolina flour, and fine corn meal as my "go to" peel dust, and I've never had any problems with the dough skin sticking to the peel if I did my part.
Tom Lehmann/The Dough Doctor