I know this is a very old thread, and I apologize for reviving it from the dead.
I just wanted to add my experience for anyone else who comes here seeking information.
I used DXM heavily when I was 16-17(I was addicted to opiates as well, and everything got out of control when my mom committed suicide)
I probably used DXM 2-4 times a week. (Stupid yes, I know, no need to preach to the choir).
Doses ranging anywhere from 350-700mg
When I stopped (which was years ago, when I went to rehab), I noticed a huge difference in my ability to speak. I have always been good with language, writing, speaking , etc. When I stopped DXM , I noticed that I had trouble forming sentences , I had trouble thinking of what I wanted to say, I even had some pretty big personality changes (the latter may be combination of my drug use, and other traumatic events that happened during those years).
I still continue to feel incredibly slow when talking to people, I have to choose my words, think of what I'm saying.
I developed incredibly bad depression, and anxiety, which still persists (I'm pretty sure that is also related to my opiate addiction as well).
Anyways, long story short, I used to be incredibly quick , witty, and smart. Ever since my recovery, I feel stupid, slow, and confused/foggy.