From the reports I read, I got the impression that the 950 penalty is not something that happens site-wide (like the +30 penalty) but rather is specific to certain searches. Have I got that right?
Yes, I would say that is correct based on the limited knowledge I have. For instance, on one site I watch, one search term turns up two pages on the last page of the search results.
While another search shows a similar page (but different search terms) on the same site at #11. The domain comes up at #1 when searched, and there is a mix of supplemental and non-supplemental pages on the site.