Any hormonal birthcontrol *can* effect milk supply. the progestroen only types effect it the least in most people, but since everyone is different, it will effect everyone differently.
Also, your supply does regulate from what it was when it first came in. When it first comes in, there is no regulation, you have no 'off' switch (not that you will till you are finished nursing), but your body learns how much to produce by what is being taken out. Having a full feeling is not good. That is what tells your body to produce LESS milk.
Your baby should be having 8-10 diapers (wet or dirty, does not matter), and a very wet diaper counts as 2. Pour 2 tablespoons of water in a diaper, and that is what is considered a normal wet diaper. As long as the diaper count is ok, your supply is fine, and it is just that your supply has regulated.
If the diaper count is NOT fine, then feel free to PM me, and I will be happy to try and help you increase your supply.
Oh, and as for my experience - I was on the mini pill for several months. It is basically the same hormones as the Mirena. It did not seem to change my supply, but I did have horrid mood swings, almost dehibilating cramps, and extremely heavy periods,(which I have never had in my life) so I decided to stop taking it.