I'm not sure from what perspective you were looking to compare asyncio to green threads (performance, readability, etc.) but there is an interesting blog post [0] by the lead architect of Twisted about the fundamental difference in programming model between explicit yielding (e.g. asyncio) and implicit yielding (e.g. green threads, regular threads). It may give you some answers.
PEP 492, from what I understood, is more about streamlining the syntax, and the major differences between asyncio (pre this proposed syntax) and green threads in terms of programming model still apply.