A lot of guitars come with plastic nuts and have tuning issues. Graphite nuts are a great upgrade - guitars stay in tune nicer and get less breakages from being pinched. Highly recommended.
You have to sand/file the GraphTech nut to the correct height - you should do this carefully and gradually rather than rush, and check progress intermittently. If you make the nut too low you can add a shim to it, but it's better to take care and be patient.