Most of the defenses he coordinated "at a pretty high level" were already at that level before he showed up, and he was only there a year or two. While our defenses were generally not good under CPJ, Collins came in and as self-appointed defensive guru, made them worse. We were notoriously bad at giving up a 1st on "Money Down".
UNC's defense was actually worse than Tech's last year, so it shouldn't take much to improve them, but they also aren't going to be nearly as good on offense with Maye and Walker gone, so the defense could be on the field even more. And I don't see where Collins has ever taken a bad defense and made it elite.
Also the last DC at UNC, Chizik, was also known as a strong defensive guy and had even been DC at UNC before improving an atrocious defense, but unlike Collins, he had actually WON a Broyles Award, won a natty as a DC, and won a natty as a HC, and helped Auburn go undefeated in 2004.
Now, perhaps Collins is better than Chizik these days and can help improve UNC. But I wouldn't count on it.