I learned some interesting wrinkles from an NAEA webinar on the new Sect 199A QBI deduction. If the taxpayer is in a specified service trade or business, or if not in a SSTB is above the ceiling ($207k or so, $415 MFJ), QBI is limited by the lower of two tests--50% of wages paid or 25% of wages plus 2.5% unadjusted basis of qualifying property. (Yes, non SSTB businesses may still get something if above the ceiling, not so for SSBT). However, if W2s are filed late with SSA, wages are considered to be ZERO for the QBI deduction calculations. If W2s are timely filed but corrected after 60 (???) days, the lower amount is used in the calcs. So if the original SSA report was lower, but it was wrong, that number still counts.
This won't affect your client for 2017, but for this year forward it might just make some of these low-paid S corp shareholders decide to pay themselves better or lose that 20% deduction they are drooling over. And just maybe it will put an end to the laggards who bring their payroll info in when it suits them. Pretty clever.