I'm surprised that changing a package parameter doesn't call the flushing proc: perhaps this is a feature and not a bug, but eventually this could be easily fixed.
I also played loading, retrieving and deleting attachments to my application objects, so hitting the content-repository-content-files folder, apparently without any error.
I want to thank everyone for giving their advice and sharing their experience. Should the clustering fail for any reason, I'm prepared to downgrade to a proven setup, with one DB server and one AS.
Meanwhile, thanks to some tuning on the VMWare side and 1 GB more on every AS, the cluster performance is now comparable to that of the physical development server, but at the same time gives me the perspective to be more scalable.