If indeed social impact bonds are allowing for the implementation of projects that would not otherwise be implemented, then you can see an immediate additionality. That's not my reading of the projects that have been implemented so far.
Addressing recidivism is a government function. Reducing recidivism, reducing homelessness, and reducing the number of children in care are ongoing programs. It's difficult to argue for additionality. If the money coming in is from the private sector, it might well be additional. If it's coming in from the McConnell Foundation, it's coming out of another arm of the McConnell Foundation, and that money would have gone into social services one way or another.
So I think the question of additionality needs to be looked at very carefully.