I really don't know that you can blame Leach's defense on him. I would think that two things contribute to their weakness on D, 1) their inability to attract a quality DC to coach in Lubbock, and 2) their inability to recruit superb athletes.
They've obviously landed a few big time athletes on offense, but for the most part, they don't recruit overwhelmingly well, due in large part to being a mid-level football program in the middle of nowhere.
If Leach were to be given a job at a school where it is easier to recruit, and a job where he could attract a decent DC, then he'd win a whole lot. A lot of coaches, and I'm pretty sure Leach is one of them, will only focus on one side of the ball and hire someone else to handle the other side. Leach is an offensive coach, so he spends his time with the offense. I'm sure he has very little input on the defense. Same could be said for Croom. Were his defenses decent at MSU? Sure, but it wasn't because he knew a lick about defense. It's because he had quality coaches on defense and was able to bring in some decent defensive talent.
And I'll bet Mullen has very little to do with your defense. He coached at UF, but his defense will likely look nothing like the defense at UF, unless his DC just happens to run a similar scheme.
And yes, Leach's scheme would work in the SEC. It may not put up 45 points per game like it does in the Big 12, but he'd put up 30+ every year, which is good enough to win a ton of games if you can hire a quality DC and recruit decent defensive talent.