Not exactly an answer to your question about the original author's experience, but there has been more detailed analysis done about XMPP on very constrained net

Not exactly an answer to your question about the original author's experience, but there has been more detailed analysis done about XMPP on very constrained networks. See for example https://www.isode.com/whitepapers/low-bandwidth-xmpp.html https://www.isode.com/whitepapers/low-bandwidth-xmpp.html