Last week I observed that the spec seems more important than the code in the world of LLMs.

Which layer is the most important?

The layer where you spend most of your time iterating.

This is especially true if there is a robust, automatic translation process to lower-level outputs.

LLMs so far aren't as robust as compilers, but if you iterate at the spec level and include lots of detail like the types of tests, you could presumably get a robust compilation.