Assuming Weapon mastery stays as it is (playtest 6), as does Shillelagh (PHB 2014), then the Shillelagh should use the mastery effect from the base weapon: the spell specifies that the wood of the q'staff or club changes its nature; the weapon is unchanged).
This means that a Shillelagh made from a quarterstaff has no Weapon Mastery benefit (quarterstaves get Flex, which would give 1d8 damage one-handed, which is what Shillelagh gives anyways), but a Shillelagh made from a club would get Slow (-10' speed when you do damage).
So make your Shillelaghs from clubs! (or tell me where I'm mistaken).
This means that a Shillelagh made from a quarterstaff has no Weapon Mastery benefit (quarterstaves get Flex, which would give 1d8 damage one-handed, which is what Shillelagh gives anyways), but a Shillelagh made from a club would get Slow (-10' speed when you do damage).
So make your Shillelaghs from clubs! (or tell me where I'm mistaken).