As @adspectus spotted and pointed out in your follow-up thread, there was a mistake in my earlier answer above:
'mime' => 'xml' indeed leads to a MIME header of text/xml, not application/xml.
However, I agree with the assessment from the other thread, that this should be ok: text/xml is a very common header for feeds (and the RSS spec only refers to the XML spec but does not command a particular MIME type itself)
For instance, kottke.org serves his using that header, and he must have tens of thousands of RSS subscribers; I’d be comfortable to follow his example 