Let's assume for a second that the intelligence is there and these models really are great. Would it make sense to hire CTO of some big-tech company to do code reviews for your startup? Feels kind of like overkill to me. Code review is not about more intelligence. To me, it's about more cultural context. And all things equal, what difference would the model make, at anything above sonnet5 medium level?

IME big models just feel like big models. There's no training or RLing a small model that will encapsulate the "world knowledge" and minutia that a big model will glance from the same training data. So it makes perfect sense to use them where "big picture" is more important - planning, code review, process review (i.e. was what was asked implemented correctly?), etc.