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As at version 2.7 the_content_rss cannot be relied upon as it does not apply to all feeds. There are bugs open for this filter which may result in it being sorted in later versions.
Meanwhile, you can apply your filter to the_content and check for is_feed() to achieve the result you are looking for.
Update: This still seems to apply for 3.0. However, from version 2.9 upwards you should use the new filter the_content_feed is in place (there is some documentation about it, but it's not complete). You can use this filter instead.