New mail node server

I have a few questions in regards to the primary & mail node servers. So if I wanted to offset the mail functionality of Server A to Server B, and currently have a fully functioning Server B with few accounts (not important ones), in order to do so I would:

• Server A: Keep the profile as "Standard"
• Server B: Switch server profile over to "Mail" profile.
• Server A: Link Server Nodes -> link to Server B.
• Switch existing accounts to distributed account (So mail goes to Server B) -> Go to Modify Account -> Linked Nodes
• Any new accounts (create a new account) would automatically have linked nodes?
• What happens to existing mailbox user had on Server A? Does users credentials for login change? Any additional things to watch for?
• From statistical point, making 2 servers one Standard and one Mail profile, does it really increase performance for web serving data (such as WP etc.) on Server A by offsetting mail functionality to Server B? Is it worth doing?
• Currently I have Server A -> DNSonly server 1 / DNSonly server 2

Any tips, help, insight is appreciated! Thanks.