I like the SSIT acronym, and haven't seen a similar formulation in the crowdsourcing literature I'm familiar with.
Generally the folks I talk to in crowdsourcing cultural heritage or natural science talk about the granularity of the task, or the granularity of the data. Fine-grained data (e.g. census entries) lends itself well to both to gamification and algorithmic quality control (by comparing double- or triple-keyed versions of the same datum).
However, the desire to leverage the benefit of SSITs can bias the materials a project chooses and its user engagement strategies. Once you get to a paragraph of text-or even a smaller unit like a sentence or address--the variations between different transcriptions of the same text become so large that it becomes very difficult to perform algorithmic comparisons. This tends to drive attention away from longer textual materials, or sometimes encourage SSIT approaches for inappropriate materials. (I remember the early FS Indexing work on Freedman's Bureau letters, which asked volunteers to tag the three most common proper names within the text, only.)
The intrinsic motivations of a user following the narrative as they transcribe can be shattered if a project forces them out of the flow of a text in order to keep them moving to the next unit of work. To see that--plus a bit of a user rebellion against statistical comparison for quality control, take a look at the "Why is there no back button?" thread on the Operation War Diary forums:
http://talk.operationwardiary.org/#/boards/BWD000000g/discussions/DWD000003lObviously I'm somewhat biased--my own FromThePage tool supports the editing approach over the indexing approach, so it's headed farther and farther away from SSITs. We are using them as fundamental data structures in FreeREG2, although I'd like to think we remain cognizant of the volunteer's desire to 'own' a whole parish register, and their wish to review and revise their work.
Ben