I don't believe in closed records, period. Nor, do I believe in closed adoptions.

I am a birthmother who was absolutely forced by my parents (who were aided by the doctor and agency...I was 16) to relinquish and was reunited after 20 1/2 years. I am going to respond to your post from my own personal experience (which I realize doesn't constitute a scientific study).

My situation has been extremely difficult. My bson's (P) aparents (G & L) were not aware that I was coerced into the adoption; they didn't find out until after the reunion (which they supported). It threatened G (the a mom). Let me also add that G and L had four bio children after receiving P (thought they were infertile....Surprise!!)

On my side, my pain, which hadn't left me for all those years, led me to make ill-thought out decisions. For example, when P pushed to meet and be part of my immediate family (which included a shaky marriage and three parented children), I gave in. For a variety of reasons that it might be helpful for me to explain later, it was a disaster.

Did I mention that P was married at reunion ? His obsession with me and the immediate family led him straight to the divorce court. After his divorce P lived in our house. More disaster.

I had problems (for years) not understanding that reunion didn't mean that I had my baby back. My husband and I had house rules and P felt that he was an adult, he hadn't grown up with us telling him what to do and wasn't going to oblige, even though he was living with us. The rules he refused to follow involved our three parented children, particularly the oldest girl. She was nine years younger. When she started high school, he was in his mid twenties but wanted to hang out with her friends and participate in the school musical. He felt it was just fine to keep my daughter, N, and her friends out past their curfews, etc.

G and L and I could not ever be friends. Oh, we tried at first. However, G and her 4 bio kids were too nervous about me (Note: I don't blame her). On my side, I was, on the one hand, grateful that they had been loving and that they had kept P safe. On the other hand, they presented themselves to a Catholic agency as educated Catholics who agreed to raise him Catholic and provide him with an education. This is why they were granted the placement. When P was 7, they joined a cult (JW), which does not support education. He spends eight hours a day going door to door, to bus stations, laundromats, etc. proselytizing. He disdains education. He is also spoiled, selfish, self-centered. He uses people horribly. He doesn't respect others' boundaries, e.g. he tried to convert my parented children to the cult. He would scream and yell at me for things like putting up a Christmas tree. It was awful.
No, there is no being friends with G and L (who are now divorced, BTW). My resentment level is quite high. They didn't do what they promised to do. I can see, too, that despite what the agency, etc. said about how much better off he would be with two parents, etc.; the fact is, he is not better off than he would have been with me. My parented kids are educated, considerate, generous, successful people.
I guess my point in all of this is to say that open adoption would have worked better for us, given the coercion, if we had had it all along (However, I would still have felt extremely betrayed by the cult thing).
I don't know what the answer is for people like us.