there's definitely measurements (eg https://hdsr.mitpress.mit.edu/pub/y95zitmz/release/2 ) but I imagine they're rare because those benchmarks are expensive, so nobody keeps running them all the time?

Anecdotally, it's quite clear that some models are throttled during the day (eg Claude sometimes falls back to "concise mode" - with and without a warning on the app).

You can tell if you're using Windsurf/Cursor too - there are times of the day where the models constantly fail to do tool calling, and other times they "just work" (for the same query).

Finally, there's cases where it was confirmed by the company, like Gpt-4o's sycopanth tirade that very clearly impacted its output (https://openai.com/index/sycophancy-in-gpt-4o/)