Somebody should jump on me and pound away for spouting things that usually turn out to be tales of old and basically incorrect...maybe I'll be lucky this time??
But I always thought speed rating had a lot to do with sidewall rigidity. By that I mean a "Z" tire will contribute less flex to the total system compliance in response to impulse (bumps) than an equal size "H" tire. A side effect would be the amount a tire "rolled over" in spirited cornering, the "Z" rolling less, and therefore maintaining better traction - all else equal of course. It would be this reduced sidewall flex that then reduced temps and therefore allowed higher sustained speeds. Conversely, a lower letter would yield a slightly softer ride at the expense of sustained speed and cornering capability. All this ignores tread design and compound of course.
BTW, I have a set of Fulda Extremos ("Z" rated, love/hate the name) that seem to be wearing well and while I don't see snow except on TV, I do get a good bit of wet driving experience and they have been good enough that I put a set of their "H" rated tires on my daughter's Lexus.