I think
Sebastian Adair is kind of over the top. It sounds more than a bit like a cheesy fictional love interest.
I'm warming up to
Sebastian on its own, though. I'm friends with one and having a real life positive assocation usually helps me get used to a name.
He gets called Seb half of the time, which helps to make it seem a little more human and less, I don't know, like a British birth announcement joining big sister
Imogen.
I might say that I like it conditionally--from the right person, on the right person, with the right middle name.