I'd count the predictive coding model and probably also GWT as larger steps than AIXI. I'm not sure where I'd put Watson.

incidentally, what did go into building it, is there a summary somewhere about what you'd want to study if you wanted to end up capable of working on something like that?

Here is a paper about how Watson works in general, and here's another about how it reads a clue. (Unsurprisingly, machine learning, natural language processing, and statistics skills seem relevant.)