I wouldn't be bothered by scale as much for a functional demo, I'd be interested in seeing my use-cases being covered and working (or the workarounds I can do to make them work).
So if large messages might be a problem, I put some of those
High throughput I show that (1000 events a second? Just throwing a number) - this also depends on what compute you are provisioning for the demo (so I'd like to see close to linear growth of capacity when adding a worker for example)
I would publish some load benchmark and how you produced it, if someone wants to challenge your performance numbers, but such a test can run in the days/weeks not something you can really show in a demo