That's interesting about the short vs. long chain inulins. None of the scientific sources I've looked at mention this overtly, nor do the places selling it in the US (it's one of these annoying ingredients that's sold as a fitness supplement, a health food, and sometimes also a culinary ingredient. So half the people selling it won't even understand your questions.
My information seems to be on the short chain variety, which may be more common here. What adds to the complexity is that (according to one study) perceived sweetness drops radically with low dilutions. So what has 35% sucrose equivalent at high concentrations may only have 10% SE at low concentrations. I imagine that with standard usage being below 4%, even the sweet variety is behaving more like 10% SE.
Also thanks again for the tip to search Italian sites. Searching for "Inulin Sorbet" turns up next to nothing. "Inulini Sorbeti" goes on for pages!