Furthermore, even if you suppose that Foom is likely, it's not clear where the threshold for Foom is. Could a sub-human level AI foom? What about human-level intelligence? Or maybe we need super-human intelligence? Do we have good evidence for where the Foom-threshold would be?

A "threshold" implies a linear scale for intelligence, which is far from given, especially for non-human minds. For example, say you reverse engineer a mouse's brain, but then speed it up, and give it much more memory (short-term and long-term - if those are just ram and/or disk space on a computer, expanding those is easy). How intelligent is the result? It thinks way faster than a human, remembers more, can make complex plans ... but is it smarter than a human?

Probably not, but it may still be dangerous. Same for a "toddler AI" with those modifications.