To me the claim that human-level AI -> superhuman AI in at most a matter of years seems quite likely. It might not happen, but I think the arguments about FOOMing are pretty straightforward, even if not airtight. The specific timeline depends on where on the scale of Moore's law we are (so if I thought that AI was a large source of existential risk, then I would be trying to develop AGI as quickly as possible, so that the first AGI was slow enough to stop if something bad happened; i.e. waiting longer -> computers are faster -> FOOM happens on a shorter timescale).

The argument I am far more skeptical of is about the likelihood of an UFAI happening without any warning. While I place some non-negligible probability on UFAI occurring, it seems like right now we know so little about AI that it is hard to judge whether an AI would actually have a significant danger of being unfriendly. By the time we are in any position to build an AGI, it should be much more obvious whether that is a problem or not.