I've hired at least 6 instructors 40 some odd years.
Some I've asked back for tuneups. A couple I wanted to dig deeper into certain topics they specialize in. Each one had a gem or 6 and each gem was well worth the entire time and $$ money commitment.
This list is not exhaustive and the benefits I note isn't to say that each one couldn't have accomplished all of them. The top guys certainly know of this stuff. I was either not listening or the topic/conversation went somewhere else. There's only so much that can be covered in 3-8hours.
Sam Elizer - Got me hooked on 3C
Harry Sims - Hooked me again after quitting the game. Fundamentals, Systems, Systems Adjustments.
Mark Wilson - Great Fundamental Lesson corrected/adjusted several things (never took lesson 2, unfortunately)
Scott Lee - Got me serious about PSR, Eye Patterns, Mother Drills......
Joe Villalpando - Predictable/Calculated Spins & increased my interest in clocking.
Lee Brett - Precision Alignment, sighting and walking the table.