You are generally right, for preceding years. As the Super was being phased out they brought the Superissimo in as the natural evolution. It's questionable whether SLX is really better than SL (which is what I think the Super is made from). From what I remember SLX is stiffer but heavier so if you're a climber you'd be better on a Super.

As for being better than a Mexico, yes it would be by some margin, the Mexicos came before the Supers. In terms of resale value, though, it's probably inverse, like a Ferrari from the 60s is worth more than one from the 70s despite being a slower car.