No one claimed gpt-3.5-turbo doesn't have any degradation over davinci-003. In fact it was quite obvious that gpt-3.5 had way less knowledge but more post trained to be helpful.

That quite strong "no one" surprised me so I checked and looking through a few blog posts from back then, they did advertise gpt-3.5-turbo as a straight up improvement and, once text-davinci-003 was to be deprecated, the instruct tuned variant as the drop in replacement [0]. If anything, they did not just promise similar performance but actually an improvement ("our best model") when compared to text-davinci-003:

> It’s also our best model for many non-chat use cases—we’ve seen early testers migrate from text-davinci-003 to gpt-3.5-turbo with only a small amount of adjustment needed to their prompts.

That's why I still remember this so well, they claimed one model to be their best and a straight up drop-in during deprecation when in my (back then even more amateurish then today) testing this was plainly not the case. A model cannot be "best" if it's measurably worse in many situations, then what was still available at the time.

[0] https://openai.com/index/gpt-4-api-general-availability/

[1] https://openai.com/index/introducing-chatgpt-and-whisper-api...