Hi,
XEmacs doesn't support Unix sockets (and neither server sockets).
Sorry. Imagine checking for GNU Emacs added to the patch.
In theory yes, but a) we also need to change the Lisp side (for N different Lisps) b) the speed difference is likely negligible as the Unix kernel should be able to recognize and optimize local TCP connections c) doesn't work on Windows d) one more thing that can be misconfigured. So, I think it's not worth the trouble.
I understand that it is not supported on some platforms. However, it is not an intrusive change because users won't randomly enter "/some/path" as slime's port number.
The speedup is sometimes negligible, but occasionally large return forms saturate the connection, in which case AF_UNIX is beneficial.
BTW, security was my original motivation for AF_UNIX, as I was not satisfied with ~/.slime-secret for a production server.