You didn't get much answer to this - I'm probably not the best person to answer, but
Are there any performance benefits to using a proxying server, or is it just for splitting mailstores?
I think this is the main reason for the proxying option. It would appear that others have measured the performance load of the proxy task and found it near negligible? Hence it seems possible to use a bunch of backend servers and a few frontend servers to forward the user to the correct backend server. I believe each connection needs to be setup each time though, so for sure some more advanced proxies with persistent caching of connections may offer a performance improvement if your servers are loaded due to the login part (but I guess measure this first before assuming it's so?)
Some have even used the backend servers as the frontend proxy servers, ie if you only have a small number of servers then make them all frontend and backend servers, eg with three servers you have a 1 in 3 chance of getting straight onto the correct server, otherwise proxy over to the correct server...
Good luck
Ed W