I don't think there's such a thing as a "typical" improvement, unless typical means "the guy reaches ~2.5x squat", since that's the squat value were more strength usually won't transfer too good to explosiveness. If you take a guy from a 0.7x squat and get him to 1.5x squat and he improves by 6 inches and stops his squat improvement to 1.5x instead of continuing to get it up to ~2.5, then how can you say that that's his near max ceiling of improvement?
So in reality you could say "your limit in terms of strength to SVJ improvement is probably a 2.5x squat" instead of throwing a number of inches or a % of the original SVJ around.
So if a guy has a 2.5x squat and jumps 26 inches, besides that he must be humongously unexplosive (and he should train that), then yes you could say that maybe the structure is very bad for jumping and therefore he doesn't have much ceiling to improvement.
But if you take formerly weak people, regardless of structure, with say a 1x untrained squat and get them to 2.5x, wouldn't you expect them to improve a lot? The only factor at that point becomes if they ARE ABLE to get to that ~2.5x number.