My 2 cents as an implementer of an alternate Python implementation -- I am skeptical about this approach. We tried the "provide a limited API" approach in the past and found the following things:

- People actually use the "hard to implement" parts of the C API

- Moving things from macros to function calls can often be quite detrimental to performance

- C extensions have been co-optimized with the C API, so changing the C API will make things less optimized

- These "check if runtime debugging is enabled" checks are not free

My guess is that this will end up in a tough middle ground where extension writers will be faced with a tradeoff along the lines of making their extension 10% slower for 99% of their users in order to make things 2x faster for 1% of their users.