That is a good indication that Mary is sinless and had been before the Angel came; but we know it from the Church, not from the scripture alone. The scripture merely confirms what we know.
We discussed this elsewhere. Being "full of grace" (which as you know is not the Greek phrase) or "highly favored" (which is much closer to the formal, "mundane" Greek) provides no evidence whatsoever of the uniquely RC doctrine of immaculate conception. In fact, in the critical text Greek, Stephen is said to be full of grace (Acts 6:8), and in Ephesians 1:6, all believers have this same exact grace, which in Ephesians is rendered as "accepted" in the Beloved. So if neither Stephen nor believers in general are categorically sinless, despite being up to their ears in grace, the Lukan passage cannot imply that about Mary either.