The 11+forum which is probably the most visited website about 11+ exams and state Grammars, takes super selective to mean a school which selects purely on score - so no catchment.
In areas which are fully selective such as Bucks, perhaps 25-30% go to Grammars, and you gain a place by passing and then based on distance. In a superselective school, there is a narrower range of ability usually - people speak of the top 5% in some. This is because more people apply per place, so the school is able to take perhaps the top 150 of 2400 applicants.
Increasingly super selectives are introducing distance criteria for at least part of their cohort. Technically, they may no longer be true superselectives. So schools which have an 'inner area' criteria won't be quite as selective as an area with no area definitions at all. However, if they still select by score within the inner area, they will be more selective than simply using a pass mark. Many of these reserve some places for out of area students and those places are truly super selective and hard to come by because applications per place are high and often only the very able even try.
Some schools like Tiffin girls and boys schools now have catchment areas but because they are very urban with large populations, still have vast numbers sitting, so can be very selective, but I guess not truly super selective because of the catchment who have a chance of a place.
The move towards adding catchments (albeit quite large ones) is I think to try and make schools more local, because fewer and fewer local children were getting into true superselectives. Also when children travel vast distances, extra curricular, parental involvement at school events etc all becomes more difficult.
Some of those that remain truly superselective with no distance cut-offs or prioritisation at like QE boys and St Olaves are always in the very top few schools. It just shows, that whilst Grammars do very well by their students usually, the level of selection plays a key role in outcomes.....not surprisingly, the cleverest children who fight off the most competitions at 11, also go onto perform best at 16 and 18, in school wide terms. Those kids would likely do extremely well in either standard Grammars or Comps, but whether they would do exactly as well is impossible to say. There is certainly kudos of going to such schools or for parents, having kids in those schools. And perhaps there is a benefit of learning with other extremely clever kids too, rather than just pretty clever kids...but perhaps it doesn't make any difference.