I don't know that I could confirm that your plant absolutely is 'Shasta' - but that is likely if it is a planted/cultivated specimen, versus a chance seedling.
'Shasta' and 'Mariesii' are the most commonly available selections in the eastern United States, both being quite floriferous and showy in fruit. There are other selections that show hints of pink in their flowers ('Molly Schroeder' being one), but that is about the only difference with many named plants.
The important note for Viburnum aficionados: most viburnum individual plants are shy to self-pollinate - meaning they are poorly self-fertile despite having perfect/monoecious flowers. You will want to have a different (non clonal) Doublefile Viburnum growing nearby that can/will cross-pollinate your plant in order to have maximum fruit display. A 'Shasta' and a 'Mariesii' make a great couple.
Fifteen feet wide? And that isn't even bragging, for northern California, I bet. Someday, someday...
