The project you are linking is great but unfortunately I am not considering importing via ACP as both environments that I need to merge have millions of nodes and several terabytes of content stores.
I tried to use Content Replication as mentioned in a first test and I actually noticed that the synchronized nodes have the same uuid as the nodes in the source environment. I have tried the various deletion and update cases and I didn't notice any particular critical issues. Is there something I'm missing that I should take note of?
Also, I know that for Content Replication to work the versions of the alfresco environments must be identical.