Citation needed :P

Unless you're using a large shared_buffers setting without configuring huge pages, you should see smaller overhead. Obviously depends on what you're doing in each connection - if you consider accessing 10k tables in each session part of the connection overhead, sure, it'll be bigger.

> introduces additional coordination overhead on queries.

It's not really the established connection that matters. The number of in-progress transactions matters considerably (a snapshot is O(#active-transactions)). There's some influence of the maximum number of possible connections.