They likely would have done slightly worse in the Big 12. It pays to be different.You don't think they could have gone 18-6 in the Big 12 over the past two years? There's not a single team that they could not have beaten either year. The real problem is that people think teams from three years ago are the same as the current team. A&M is nothing like their former self, and using their success with Manziel/Sumlin compared to the trainwreck of Mike Sherman is flawed completely.
You say there is less defense in the Big 12, but we have seen with A$M and MU's success, it is more of a case that there is more offense in the Big 12. In the Big 12 teams have the offense to capitalize on average defenses. If A$M or MU have an off day or injury on offense they cannot count on the other team having a bad offense (unless playing The Mess).