I have had people tell me in the past that playing at HU table or HU SnG will help me improve my late stages tourney play and HU at the end of MTT or 9 seat Sngs.

I really didn't think it was that useful for me. I always feel that the play that leads up to HU can have more impact on the HU play at the end. While there are aspects of straight HU play that can help, I've always felt player play observations more useful.