IMAP IDLE scalability

Is there a doc out there somewhere that talks about the scalability  
of the IMAP IDLE implementation? I've never dealt with enpd so I  
don't know how well it scales.

So far I'm leaning towards running one enpd on every mailstore we  
have rather than running one that is responsible for all mailstores.  
Currently the entire deployment sees up to 600,000+ simultaneous  
connections at a time. Which is spread over several stores that  
handle 12,000+  of those connections each.

But, if I do expose IMAP idle I expect that number will go up some as  
clients that make use of idle will well, idle rather than  
disconnecting and then reconnecting later.

Or, it could stay the same as the clients would have a persistent  
connection open anyways that sits there and does status commands  
every now and then.