The real URL should also work, but for some reason, the Itemid is causing it not to. In JoomSEF's Advanced configuration, make sure you have both "Set server QUERY_STRING" and "Prevent non-SEF variables from overwriting the parsed ones" options enabled.
If that doesn't help, you can disable SEF for feed URLs, if you open Content extension parameters from Manage Extensions page in JoomSEF, and there on Common tab set "Do not SEF URLs matching regular expression" option to "format=feed" (without quotes).