But if she was sinless she had the same nature as He did.All she would have added was a body
I don't see how that follows.
Mary is a creature. Ab initio creatures are essentially (and "naturally") different from the Creator. Sinless or not, she is still a creature.
If we are going to say that sin is a necessary aspect of creatureliness, so that to be without sin is to be Divine, then we vitiate the Fall story, I think, and change the meaning of God's frequent assessment of His work as "good" and "very good."
Further the saints in heaven are, I imagine, sinless. That does not make them divine.
So it does not follow the being preserved from sin at the moment of her conception (as we hold) makes her any less of a creature or any more divine.
To be my usual fatuous self, I can imagine the famous and oft parodied "I found you in the gutter, and I took you, and molder you, and made you a STAR!" line, and apply it to Mary. She is a sign of the great things God can do and will do when He starts with a human type personnel.
Leaving aside the good or bad reasons for what we hold, it's spinning our wheels to suggest carelessly that we hold what we do NOT hold. We teach that Mary is an eschatological sign. In her are "currently" fulfilled what all the blessed may hope to enjoy. It says that because we see Him we shall be LIKE Him. I do not think that means that our creatureliness will be totally negated.