Although you might get some duplication of files, typically most things backed up as part of the System State or Shadow Copy Components are automatically excluded from flat file backups via the drive letter using a process known as Active File Exclusion - this process also stops things like the swap files (and if present database files that should use a specific agent) being backed up as well.
Because of this I would not expect you to gain that much back on your total backup size by trying to locate individual items to exclude.
Just my 2p