I would say there's a small minority of our changes that we can upstream, and eventually we'd like to upstream them. For the rest, I would say the different set of priorities for the two projects means they probably will stay separate:

- In the blog post I linked to an issue of someone trying to upstream a quickening implementation and meeting resistance due to the added complexity

- CPython prefers portability over performance, and we added a number of big build dependencies that may not work on all the platforms that CPython supports, though from our perspective it works on all the important ones

- CPython has included a number of performance-degrading features over the years and we plan to start cutting out some that are only used for debugging but still hurt release mode. I don't know for sure but I would expect resistance to backing out those changes.

There are more, but the general idea is that the two projects will make different tradeoffs. Maybe there is a world where the code all lives in the same repository but is gated between different behaviors, but the CPython maintainers have already rejected a proposal like that.