Context
Let's assume you have a server which exposes a web server and one or more web services to store and manage sensible information about physical persons (assume, for this example, complete medical history but also phone numbers, e-mails and other private information).
Access is authenticated and you did in your code everything you need to reach a reasonable level of security.
Web server and web services are running on a Windows Server with IIS + ASP.NET and databases are in a single SQL Server instance. Assume system is always up-to-date, logs are carefully evaluated and system is properly configured and an attacker has no physical access to machine itself.
Current Architecture
SQL Server is installed on a separate - firewalled - machine (What is the best practice for placing DataBase servers in secure network topologies and How do you explain to experts that a database server should not reside in the DMZ?).
Obviously every user input is validated and sanitized (if/when required), also information sent by client (even when not directly entered by user) are re-validated and inconsistencies trigger alerts).
Even if not directly related to DB also remember that:
Proper password managing (store hashes with a good - and slow - hashing algorithm, also described How to securely hash passwords?) and security rules (passphrases are encouraged over short/complicated password and password changes are required only after many failed login attempts, see also How does changing your password every 90 days increase security?).
Handling of parallel attacking (incremental delays for each failed login - both from same IP and for same user name - and black lists). Related: How to uniquely identify users with the same external IP address?.
Sessions have timeout (user activity reset short one, long one is fixed). Also client side there is a weak protection that automatically disconnect user (same described in Google Chrome restores session cookies after a crash, how to avoid? but also to automatically log-out when navigating away) if he navigates away without disconnecting.
Logs are manually monitored but there are rules that automatically trigger alerts.
Data are stored in N+3 different different databases (each one configured with exactly required permissions, no sa-like access, as described How to secure database table of users for an application?).
One database for logs (write-only for web server accounts).
One database to store login information (readonly for web server accounts, a different Intranet web application will run with a different user).
One database to map login with a physical database (and other internal stuff to manage accounts), again read-only for external accessible web application.
One separate database for each user of the system (read and write for everyone).
Question
To use three different SQL Server instances (one for logs, one for accounts and mapping and one for all user databases) will increase security or just complexity?
Will this also affect performance? (If you can't answer this without more context, you may simply ignore performance issues unless they're obviously much worse)
Moreover is there any drawback to merge together mapping information and accounts? (separate databases with same permissions will increase security in any way?)
Considerations
I know that in security often "more is better" (at least it's a common motto) but drawbacks may be greater than any benefit (if any):
Increased cost for hardware and software.
Increased complexity (both for setup and maintenance), this is IMO a big drawback because a (possibly) safer system with a non-optimal configuration may be much less secure than a simpler one.
My perplexity is because if an attacker will be able to run arbitrary code (because of a bug in my application or because of an exploit) then it doesn't matter where things are: it has all resources to do what he wants, I assume we won't detect attack quickly enough to stop service then time he'll need to understand there is another machine to connect to is small compared to total time he has to perform his actions.
I don't know if this question is strictly on-topic here, it seems to span across multiple SE sites and I'm not sure which one is right one.