That's true, the 2004 5.1 mix is the worst soound mix I have ever heard in my life, sounds like it was created by randomly taken lines from randomly selected sound mix
Your method is interesting, but do you know some software pro-logic decoder ? Also on doom9.org you can find a lot of tutorials on how to make a 2.0 -> 5.1 "upmix", but that's not a "true" 5.1 mix