Are there plans to elucidate implicit GC costs as well?

Great question! I actually just touched on this in another thread that went up right around the same time you asked this. It is clearly the next big frontier!

The short answer is: It's something I'm actively thinking about, but instrumenting micro-level events (like ZGC's load barriers or G1's write barriers) directly inside application threads without destroying throughput (or creating observer effects invalidating the measurements) is incredibly difficult.