Found the issue responsible for the race-y failures with the RemoteMessageThroughputSpecs:

https://gist.github.com/Aaronontheweb/4d19314abff6d8efa31f

It appears as though the EndpointWriter can get it itself into a state where it endlessly buffers messages on the outbound side of an association, depending on a race condition with the speed at which the association forms.