I'm taking a roughly similar strategy now. Previously I had many of these:

is_xml $result, $expected,
  '... and we should receive the correct XML';
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By generating $expected with a different algorithm (to ensure that I'm not just reaching into buggy code), I had to constantly maintain two different sets of code which did the same thing. It was very painful. Now I'm doing this:

is_well_formed_xml $result,
  '... and we should receive well-formed XML';
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And to verify that it's correct, I'm adding more high-level integration testing. It's an annoying trade-off, but like what you're suggesting, it's a reasonable one.