While I am teaching to use AI coding assistants (and emphasize the pros and cons, in particular that it helps only if you have enough knowledge to judge whether the output is correct or pure hallucination), there have been recently two studies published that give an indication that AI coding assistants do actually decrease productivity:

• Measuring the Impact of Early-2025 AI on Experienced Open-Source Developer Productivity
• Where's the Shovelware? Why AI Coding Claims Don't Add Up

One aspect is that developers learn over time about the project that they are working on whereas LLMs have been trained once and will always start from scratch in a project where they are used as coding assistants.

A personal idea for future research: as there have been reports about AI slob (e.g. AI generates low-quality documents that get distributed and then take time to read and therefore, this reduces in the end productivity), it might be worthwhile to measure the impact of AI coding assistants on maintainability: if AI coding assistance would produce code that is hard to maintain, then any gains by AI coding assistants might get eaten up in future when it comes to maintenance.