Is there any disadvantage to a plan sponsor to making the top-paid group election to determine HCEs? The only one I have been able to think of is that it introduces some additional complexity to the HCE determination, conceivably raising the chance of errors. Are there any other disdavantages?

I guess I wasn't thinking. Of course, at least in a 401(k) plan, whether or not the top-paid group election is helpful or harms the plan depends on the amounts the participants who would be changed from HCE to NHCE by the election are deferring. Anything else I should have thought of?