The exact-search baseline should separate approximation loss from representation loss. In your example, both the indexed query and the forced exact scan operate on halfvec(1536), so the comparison measures ANN recall within the already quantized space. It does not show whether float16 storage changed the neighbors relative to the original float32 embeddings.

I would report two deltas: original-vector exact search versus halfvec exact search, then halfvec exact search versus its HNSW result. That would make the memory-saving decision independent of the index-tuning decision, particularly for nearly tied neighbors.