I don't think I follow. This isn't just a precision issue, but completely garbage output, right? If that's the case, I think it's better to have a separate helper like check_PropVariantToBSTR2() so that we don't need to spread this all over, and it will be less noise when we'll start getting rid of win7 workarounds.
But in general it definitely valuable to clear at much failures as we can from tests page.