Using an sftp package server, PORTAGE_SSH_OPTS works for options such as "-i /path/to/my/key" but it does not work for either "-P 2222" or "-oPort=2222". I confirmed in the firewall logs of the package server that emerge is attempting to connect to the sftp server via port 22, and not the port specified in PORTAGE_SSH_OPTS.

Portions of my make.conf:

PORTAGE_BINHOST="sftp://packages@my.server.com/packages"
PORTAGE_SSH_OPTS="-P 2222"
#PORTAGE_SSH_OPTS="-oPort=22"
#PORTAGE_SSH_OPTS="-i /path/to/my/key"
FEATURES="${FEATURES} getbinpkg"

Note that uppercase P is correct for sftp (lowercase p is used for ssh).