The C API exposes implementation details for historical reasons (there was no design for the public C API, the public C API is just the private API made public), but also for performance. Macros are designed for best performances, but should be reserved to developers who have a good understanding of CPython internals.
Hiding implementation details is likely to make tiny loops slower, since it adds function calls instead of directly accessing the memory.
The performance slowdown is expected to be negligible, but has to be measured once a concrete implementation will be written.
Question: would it be acceptable to have a new better C API if the average slowdown is around 10%? What if the slowdown is up to 25%? Or even 50%?
Right now, the project is too young to guess anything or to bet. Performances will be carefully measured using the Python benchmark suite pyperformance, but only once the design of the new C API is complete.