As a previous poster has said, ready made polenta is syn free.
The debate around ground polenta, or cornmeal, comes because the syn bible says it is syn free but when used as a flour, it has a syn value.
So what constitutes using it as a flour? That's the 64 million dollar question that we haven't had a definitive answer to.
For your recipe I would say it is syn free. You are using the polenta for the purpose that was intended. There is a recipe in one of SW books for cornbread which also takes it as syn free. Again that is using polenta as intended.
Where you use the cornmeal to make muffins then that I think is technically using it for a different purpose, and it becomes synnable. That said, personally my view is that as long as you're not having them every day and you continue to lose weight, then go ahead and enjoy the muffins. And as others have said, a polenta muffin has to be better than a full-fat muffin.