You _most certainly_ have false postives if you block \200 - \377.
it's certainly the case that *I* have no false positives when filtering on
unencoded non-ascii characters in the message header. any mail that I get
that's not in English and is not encoded correctly isn't something that I need
to read - assuming I would even be able to read it, which is unlikely.
that and there's such a thing as "too broken to deal with". untagged,
unencoded 8bit text IMHO clearly qualifies. I recognize that others' opinions
may vary, and how you configure your MTA is your own business. But I don't
see any obligation for MTAs or UAs to deal with whatever random trash is
thrown at them.