Defense is a strange beast in general in these games. Scheme and Talent go hand in hand, it also is nearly impossible to truly gauge different era's against each other due to changing offensive trends.
Gailey's years were the years the ACC was actually viewed as a defensive conference. Most teams were running some form of power running game, 14-7 games were common all over the league. We ran an attack first defense designed to blow up running games. Tenuta's defenses were extremely vulnerable to spread passing attacks see Matt Ryan but we didn't play those kinds of attacks that much. So Tenuta's numbers actually benefit a lot from the fact that the ACC in general was not a high scoring league.
for example in 2006 here is the Points per game stats
https://www.teamrankings.com/college-football/stat/points-per-game?date=2007-01-09
notice the only ACC team in the top 10 is clemson, then you have to go all the way to 38 to get to florida state. The two teams that won the divisions this year are outside of the top 50. This was a very different ACC. Just 2 years later we have CPJ, Cuttcliffe, Dabo taking over. And a lot of schools switch to spread attacks.
Spread offenses are designed to isolate and attack individual matchups. You don't have to beat the entire defense... just one guy. They are designed to equalize talent disparity whether they do it through the option like we do or through spreading the field and throwing quick game passes and using RPO's to create isolation. In this universe you have to approach the game very different. PJ hired wommack who did an OK job against these teams with NFL starter caliber defenders. Groh was a miss completely. So he went with Roof after.
Roof faces a very interesting problem with the ACC. At GT we judge our success two ways, acc titles and record against UGA. As much as people talk about how unique our offense is to deal with. In the terms of the ACC, uga's offense is anachronistic. The only team that runs anything close to a Pro Style attack that we play regularly is Pitt. Everyone else is the spread. Its up tempo offenses, its no huddle attacks designed to isolate defenders. Roof's solution to this problem was a very read and check heavy defense to allow the players on the field to get into the right play based on formation and film study. To generate turnovers. The shift to a base 4-2-5 is to help counter the personnel mismatches that happen against spreads ( you don't want line backers on 4.3 slot receivers.) Unfortunately such defenses require exceptionally long hours in the film room to get. Our athletes have less time for that especially during Midterms and other academic milestone weeks. Which also happens to be right around the time we typically "simplify the defense and get better". Roof is addressing that this year. Which is year 5 and the first year that most of the defense is comprised of his recruits.
A defense designed to counter the Spread has troubles with power run games. the uga game is a completely different game to call and we have actually played better defensively in these games since Roof has been here. That kind of defense is his comfort zone. our two losses
2013 - Vad goes turnover happy, offense can't hold on to the ball, defense that lacks depth gets tired stopping Todd Gurley an nfl starter caliber back.
2015- We can't do anything on offense. Defense holds the dwags to 13 points with their only touchdown coming off a 4th down run where we sell out to stop the run and get beat.
Compare that to 2009 where we can't stop the run to save our life ( if you want to know why wommack got canned it was this game)
So has roof underperformed? Yeah I think he has but it isn't because he's stuck in his ways and isn't adapting. The only missing piece from the Roof defenses is nfl caliber talent on the d-line at the DE position generating pressure on the QB with 4. Once you have that, the blanket coverage read and react approach is the text book way to stop a spread. This is the Show year.
