You can't measure productivity for shit, otherwise companies would look entirely differently. Starting from me not having to do my own finances or event planning or hundred other things that are not my job description, not my specialty, and which were done by dedicated staff just a few decades ago, before tech "improved office productivity".
> We aren't seeing the developers who start using copilot/gpt rush ahead of their peers.
That's because individual productivity is usually constrained by team productivity. Devs rushing ahead of their teammates makes the team dysfunctional.
> We aren't seeing any ability to cut back on developer spend.
Devs aren't stupid. They're not going to give you an opportunity if they can avoid it.
> We aren't seeing anything positive yet and many developers have been using copilot/gpt for >1 year.
My belief is that's because you aren't measuring the right things. But then, no one is. This is a problem well-known to be unsolved.