No such studies can exist since AI coding has not been around for a long term.

Clearly AI is much faster and good enough to create new one-off bits of code.

Like I tend to create small helper scripts for all kinds of things both at work and home all the time. Typically these would take me 2-4 hours and aside from a few tweaks early on, they receive no maintenance as they just do some one simple thing.

Now with AI coding these take me just a few minutes, done.

But I believe this is the optimal productivity sweet spot for AI coding, as no maintenance is needed.

I've also been running a couple experiments vibe-coding larger apps over the span of months and while initial ramp-up is very fast, productivity starts to drop off after a few weeks as the code becomes more complex and ever more full of special case exceptions that a human wouldn't have done that way. So I spend more and more time correcting behavior and writing test cases to root out insanity in the code.

How will this go for code bases which need to continuously evolve and mature over many years and decades? I guess we'll see.