I changed the owner/group of the wp-config.php file to www-data (Apache's user).
I changed the file permission to 0500.
Is that more secure than leaving my normal user as the owner and file permission of 0755?
"More secure" is relative - making apache the owner opens the file for potential remote access more so than if it were owned by your user. After all, I assume there's no process listening on an open socket running with your user credentials?
Pedantry aside, the file permissions themselves could preferably be 0400, as apache/mod_php only needs to read and not execute the file.