I have been utilizing this same feature for a year now. Works like a charm. No rollover fees, limits, check/letter writing, or need to roll over after-tax earnings into a Traditional IRA as none will exist. Taken from my 401k plan:
“Automatic After-tax Roth In-Plan Conversion” feature, activated by calling. With this feature, After-tax Contributions are automatically converted to Roth contributions on a daily basis (i.e., before those contributions accrue any taxable earnings). "
See picture for how your contribution will show on the Fidelity website: