My understanding is that can is only half the question. Yes, the changes CAN be upstreamed but my understanding (through others, so take it with a grain of salt), is that the CPython doesn’t like to take optimization changes that obscure the CPython code base. As a result, they often deny merging optimizations in favor of readability in the C codebase. Again, just my understanding and may not be 100% accurate.

Cinder is fully compatible with the CPython's C-API