emerge is not using FETCHCOMMAND_SSH for downloading Packages file: if port is not set in PORTAGE_BINHOST, it will respect /etc/ssh/ssh_config. But when downloading .tbz2 files emerge uses FETCHCOMMAND_SSH and if port is not set in PORTAGE_BINHOST, it will force port 22 instead of respecting /etc/ssh/ssh_config. I think FETCHCOMMAND_SSH should use ssh_config ports by default instead of forcing port 22.