I don't think the extra width gains you much on the track, unless your track is extremely bumpy (may be the case in the US). I think the bigger concern for trackies is the aero profile of tire width + rim width/shape. E.g., if you have an older Io, you probably still want to run a 19 on it.
EDIT: By the way, Marymoor is one of the smoother tracks in the US in my experience. Not as good as Carson, on par with Trexlertown and Colorado Springs, certainly better than Hellyer and Washington Park Bowl, and probably better than most others.