Not sure who your service provider is but RIM will tell you, "yes it may work but officially, we don't support multiples sites per BES" Documentation for it would be found at
http://www.blackberry.com. And yes, I'd be pretty sure your delay is do to bandwidth issues. As I stated earlier, MAPI is very chatty and it is not the only traffic going through your pipe so it could be slow depending on what you are running through your pipe.