The instructions for Line 8 of the 1120-S state:

Don't include salaries and wages reported elsewhere on the
return, such as amounts included in cost of goods sold, elective
contributions to a section 401(k) cash or deferred arrangement,
or amounts contributed under a salary reduction SEP agreement
or a SIMPLE IRA plan.

But then for Line 17:

Enter the deductible contributions not claimed elsewhere on the
return made by the corporation for its employees under a
qualified pension, profit-sharing, annuity, or simplified employee
pension (SEP) or SIMPLE IRA plan, or any other deferred
compensation plan.

The way I read this is that if the $8,000 (from your example) comes out of the employee's wages, you include it on Line 8, which would show the FICA (or higher Medicare) wages from the W-2.  But if the company then "matches" this with another $8,000, that would go on Line 17.