The 80/20 research seems OK to me, but I'm not a professional researcher. For example, an OISE study of 1,500 adults found: "Approximately 70 percent of Canadians say that their most important job-related knowledge comes from other workers or learning on their own rather than employment-related courses."

Will, I don't quite follow your questions about modification. My approach to informal learning is 1) that it happens, 2) that most experienced workers know what's best for their job context if they have been provided with correct and current information, and 3) that supporting informal learning so that some of it can be captured and shared is usually beneficial to individuals and the organization.

An HPT approach would be that formal training addresses a lack of skills & knowledge but there are other barriers to performance that training can't help. Two of these are lack of information and lack of sufficient resources. I include job aids, communities of practice, journals, blogs and wikis in the possible suite of informal learning tools: