I'm thinking @velo has the most likely answer. ie cage is bent. Sometimes it's easier to remove the RD to eyeball alignment of the pulleys.
2nd thing is as you have already raised - the hanger. And it's true to say that a lot of hangers are not straight.
It needs to be checked with the proper tool. My only caution here is that some hangers don't take well to "adjustment". The metal is often a brittle alloy that can be weakened by the process.
There's sometimes a buggy.
How many drivers does a buggy have?
So let's just say I'm drivin' this buggy...
and if you fix your attitude you can ride along with me.http://www.youtube.com/watch?v=GekiIMh4ZkM