It is really hard to tell with McClay, because we don't actually know what are his moves and what aren't. I wasn't actually sure when he came aboard so I went to the Cowboys team page to see. I went to the "Coaches & Executives" page and it was weird, because McClay is listed nowhere on that page. It has all the coaches, and all the Jones family...but no McClay.
His job description is that he basically is a unifier within the organization, working to get everyone on the same page, from Jerry all the way down to each scout. He supposedly is a non stop machine of reviewing players, searching for fits for the Cowboys. His biggest activities seem related to free agent acquisitions but he apparently has some say in the draft as well. The problem is when evaluating him you don't necessarily know when he deserves blame or praise.
I think the first draft he was involved in here was the 2014 one, which was pretty decent at the top with Martin, Lawrence and Hitchens. Not much after that with only Terrence Mitchell playing still as far as I can see. THe 2015 draft has pretty much turned into a stinker. Byron Jones has not made the switch from athlete to football player and likely is in his last year with the team in 2018 unless something changes. Randy Gregory, well, let's just say whoever believed he could put the weed down got played. Chaz Green looks like he will be out of the league soon, and Damien Wilson is mediocre to average at best. Swaim has hung on and I guess for a 7th rounder has been ok, but overall this draft is not so hot. I would say one more year to evaluate 2016 but so far so good. The Jaylon Smith risk probably wasn't his doing. Same with evaluating the last draft, but initial signs look pretty promising.
If free agency is McClay's baby, then it is hard to see what the hype around him is for. He initially was involved in researching pro personnel around the league and an article I found said his best additions he brought in include Laurent Robinson, Tony Fiammetta, Ernie Sims, George Selvie, Nick Hayden and Rolando McClain. I'd go 50-50 on that group as being positive, with Robinson, Selvie and McClain being good additions. Our offseason free agent acquisitions, mainly lately, have been awful. Nolan Carroll, Cedric Thornton, Stephen Paea, Robert Blanton, Damontre Moore, Jonathan Cooper, Byron Bell, Joe Looney, Benson Mayowa, Alfred Morris, Darren McFadden, Andrew Gachkar, Greg Hardy and Corey White are probably the best known. Of these, the only ones worth a dang while here are Cooper, Morris, McFadden and maybe Hardy. Not exactly a record to be exceedingly proud of. The one thing to consider is that these have all been "value" picks for the most part, guys that for one reason or another where cheaper than they might have been if not for certain issues.
So, trying to determine just exactly how important McClay is really isn't very easy. The Cowboys organization makes it hard to put the blame, or praise, on just one person.