Mass Spell Recursion & Storm
The spells cast by Mizzix's Mastery would count towards the storm count if you casted them in the correct order.



From the Gather rulings under Mizzix's Mastery:


  If Mizzix’s Mastery exiled multiple cards, you may cast the copies in any order. The last copy you cast will be the first one to resolve.


The correct play to maximize your storm count would be to cast every spell other than Grapeshot first, and then cast Grapeshot last.  

For instance, lets say your graveyard had Time Walk, Ancestral Recall, and Grapeshot, and you had casted no other spells that turn.


You cast overloaded Mizzix's Mastery (Storm=1) 
Each player passes priority, Mizzix's Mastery resolves.
Exile Time Walk, Ancestral Recall, and Grapeshot, making copies of each of them
Cast the copy of Time Walk (Storm=2)
Cast the copy of Ancestral Recall (Storm=3)
Cast the copy of Grapeshot
Grapeshot's Storm ability triggers on cast with the Storm count equal to 3
Place 3 copies of Grapeshot on the stack on top of the original Grapeshot
Each player passes priority, Grapeshot storm copy 3 resolves
Each player passes priority, Grapeshot storm copy 2 resolves
Each player passes priority, Grapeshot storm copy 1 resolves
Each player passes priority, Grapeshot Mizzix copy resolves
Each player passes priority, Ancestral Recall copy resolves
Each player passes priority, Time Walk copy resolves