At least as of 24.3, the manual[0] suggests that such a conversion
should not occur in this case:

    You can also use hexadecimal escape sequences (`\xN') and octal
    escape sequences (`\N') in string constants.  *But beware:* If a
    string constant contains hexadecimal or octal escape sequences,
    and these escape sequences all specify unibyte characters (i.e.,
    less than 256), and there are no other literal non-ASCII
    characters or Unicode-style escape sequences in the string, then
    Emacs automatically assumes that it is a unibyte string.  That is
    to say, it assumes that all non-ASCII characters occurring in the
    string are 8-bit raw bytes.

[0] (info "(elisp) Non-ASCII in Strings")

Josh