After 6 months, your employer can more accurate judge your work and it isn't necessary to wait until the "annual" review. It's an opportunity to adjust salary upwards because you've proved your value. Remember, the original negotiated salary was based on pre-job leverage (skills, knowledge, etc) and not how well you exceed in the role. You could prove to be a bigger value-add than was expected.

You don't have to pose it as an ultimatum (Pay me more or I'll quit), because you can also use it as an opportunity to find out how to adjust in order to get the raise at the 1 year mark, which is in another 6 months.