I think this is a very hard thing to quantify. How do you separate out objective observable behavior from subjective experience?
What I mean is .. a client that has an internal need to feel special to the therapist (I am not being judgy about that at all, some people have a genuine need for that) will interpret the therapists actions through that lens.
A client who has a dismissive attachment style might interpret the same objective T behavior in a very different way.
Some therapists likely do actively try to make their clients think and feel their relationship is special. Again different clients would interpret the same behaviors differently.
I am just pointing out that it will be impossible to tell which "side" the intention and experience of the specialness of the relationship is coming from. For some people who respond yes it will be purely the therapists action, for some it will be purely the clients interpretation, and for others it will be a mixture of both.