It may seem strange, but the amount that you contribute to the Roth 401(k) has no effect on the amount that you are eligible to contribute to an IRA. Only pre-tax elective deferrals to the traditional account in the 401(k) reduce the amount of wages available to contribute to an IRA. In your example, $6,000 could be contributed to the Roth IRA (modified AGI permitting), no matter how much of this same income is contributed to the Roth 401(k).
