Would it make security sense to split up tables into different databases and or servers?
Not knowing what system you are using I can only make a couple of general observations.  If you have multiple tables in a database then the administrator of the database will have access to all of them but otherwise you can control the table level security.  If you have multiple databases on a server then the administrator of the server will have access to all of them but otherwise you can control the security as you desire.  The best reason to split the data up between servers is to limit the access of an administrator on the server.  

At least in MS SQL Server (the DB I know the most about) you can locate tables across multiple databases on the same instance with no performance issues.  Once you are dealing with multiple instances on the same server you have a distributed transaction and may or may not take a performance hit.  If the instances are on separate servers in the same location then you add in performance degradation caused by the LAN. And of course if you have servers in different locations you are dealing with WAN performance.