After tax contributions in small S.H. 401(k)

Owner and his wife max out on pre tax deferrals. They fund the basic safe harbor match.  He exceeds 401(a)(17) and she makes $48K.  They are considering amending the plan to add after tax deferrals.  He read a Wall Street Journal article and the thought of exceeding the max up to $73k per couple is enticing.  They do not put in a profit sharing contribution.  Does the after tax contribution negate the benefit? 

Are there other employees who qualify for the plan?

There are 16 active participants including the owner and his wife.  The plan just went top heavy for 2018

Voluntary after-tax would be subject to ACP testing, so unless they had substantial rank and file after-tax participation it wouldn't work for them. This is a great strategy for solo/owner-only/HCE-only plans and possibly very large corporate plans that are already easily passing ACP testing.

in addition, a plan receives it's "Get out of top-heavy requirement" card if the only contributions are deferrals and safe harbors. the general opinion is that after tax ruins this feature. in your example, a plan with a safe harbor match needs to kick in 3% to any participant active at the end of the year not deferring. And yes, even someone like me who might not be considered 'active' in regards to work on the job, as long as employed on the last day, as a participant would receive top-heavy. I know, such a broad definition of 'active', but that is the way it goes.

Thank you for your comments. This is helpful