Well it's all tradeoffs, right? 6 months for 9 FTEs is 54 man months. 2 months for 2 FTEs is 4 man months. Even if one FTE spent two extra months perusing every line of code and reviewing, that's still 6 man months, resulting in almost 10x speed.

Let's say you dont review. Those two extra months probably turns into four extra months of finding bugs and stuff. Still 8 man months vs 54.

Of course this is all assuming that the original estimates were correct. IME building stuff using AI in greenfield projects is gold. But using AI in brownfield projects is only useful if you primarily use AI to chat to your codebase and to make specific scoped changes, and not actually make large changes.