I think any evaluation of Ted Roof's defense shouldn't even begin until mid-season of 2014 because he didn't take over as our Defensive Coordinator until 2013 so there were things that needed to happen before we would even have a clue what it might even look like.
For example, Al Groh had recruited players for his base 3-4 defense and Ted Roof plays, generally, a 4-3, so not only are the number of position player different, but typically, physical attributes and playing styles (gap attacks, etc) are different. so there had to be a "bell curve" in ramping up that component of the defense (physical).
Next, the players also were had to learn a whole new defensive scheme than they had been playing over the past several seasons, so they were playing "slow" initially because they were often-times spending a lot of time thinking about where they needed to be more than reacting quickly naturally because it was more second-nature as they had practiced and knew their assignments so well. This also gave TR a comfort level that allowed him to become more aggressive in his play calling, such as throwing in more blitz packages, which led to more turnovers the second half of the season, and holding teams better defensively the second halves of games because of great adjustments at halftime.
While it's true we'll be losing some key players, like Neally and I. Johnson, from the defense, we're still getting back a vast majority of the starters, and several experienced subs, as well as this coming Fall (2015), even more of TR's personal recruits will be on the field (and those who have been effectively moved into new positions sine AG has been replaced), and the defense will have had over 3 years learning his base defense, so they will be playing even more instinctively, and he should be even more aggressive in his play calling. This should mean our defense should be even better than the second half of last season.
If this is, indeed, the case, and we can have our new RB's and WR's step up to the plate, like they have in the past when previous starters have graduated, then GA Tech's football team should be poised for a special season in 2015 because they'll be going into the Fall ranked in the Top 25 and have a schedule that is set up to allow them to participate in the College Football Playoffs if they can win their games.