I can think of a counterpoint or a workaround (or at least a sketch of either) for each of your objections, despite being moderately intelligent for a human. A superintelligence will think of many, many more (if it couldn't, it wouldn't be much of an intelligence in the first place).
For example:
> The parent was positing, at a minimum, a 30 million fold increase in clock speed. This entails a proportional increase in energy consumption, which would likely destroy any thermal envelope the size of a brain.
No reason to restrict the size of an artificial mind to that of a human brain. With no need to design for being part of a mobile, independently-operating body, the design space grows substantially. Even on existing hardware, you can boost the serial processing speed by a whole order of magnitude if you submerge it in liquid nitrogen. I imagine you could get much greater speeds still on hardware designed specifically for cryogenic cooling with cryo liquids at much lower temperatures. Maybe not 7 orders of magnitude difference, but 3 seem plausible with custom hardware.
Corporations are a good example of what's effectively AIs living among us today, but their performance is indeed bottlenecked by many things. A lot of them don't seem fundamental, but rather a result of the "corporate mind" growing organically on top of the runtime of bureaucracy - it's evolved software, as opposed to designed one. E.g.:
> there are many situations where the steps to design a solution are sequential and a hundred people won't get you there any faster than two
That's IMO because managers and executives, like everyone else, don't see employees and teams as a computational process, and don't try to optimize them as one. You can get a boost on sequential problems with extra parallelism, but it would look weird if done with human teams. You could absolutely do pipelining and branch prediction with teams and departments, but good luck explaining why this works to your shareholders...
The point being, while corporations themselves won't become superintelligences, due to bottlenecks you mention, those limits don't apply to AIs inhabiting proper computers. 7 orders of magnitude advantage over humans may be way too much, but I don't see anything prohibiting 2-3, maybe even 4 orders, and honestly, even 1 order of magnitude difference is unimaginably large.