We have a feat called Shield Master in Beta already, so I feel Sheild Mastery, and Greater Shield Mastery are names that could get confusing.
I suggest that since they are being coupled with Shield Focus and Greater Shield Focus, that the word Specialization should be used instead of Mastery. This would match the precident with Weapon Focus, and that chain of feats.
So the tree would become:
Shield Focus -> Shield Specialization -> Greater Shield Focus -> Greater Shield Specialization
Far less chance of confusion with Shield Master.
