There are a very light benchmarks in the Readme, or are you looking for more?
Which doesn't address the question: do LLMs understand TOON the same as they would JSON? It's quite likely that this notation is not interpreted the same by most LLM, as they would JSON. So benchmarks on, say, data processing tasks, would be warranted.
[0] https://github.com/johannschopplich/toon?tab=readme-ov-file#...
1. Retrieval Accuracy - https://github.com/johannschopplich/toon?tab=readme-ov-file#...
2. Performance by dataset - https://github.com/johannschopplich/toon?tab=readme-ov-file#...