I think calling Aggy the "Auburn of the Big 12" is a pretty generous reading of where they were when the left.
Aggy hadn't sniffed the Big 12 title game in more than a decade by the time they left the conference. They made the Big 12 title game in 1997 and 1998 before Stoops took over at OU and before Leach got TTU going and Miles/Gundy got OSU going. Once those three things happened, they never sniffed it again.
Their last 5 years in the Big 12 their records were:
7-6, 9-4, 6-7, 4-8, 7-6
They had once decent year along with two seasons with losing records and two 7-6s. They were clearly behind OU, Texas and OSU/Tech in the Big 12 South pecking order.
If we play out what happened over the next decade, it's pretty clear they would have remained the 3rd or 4th best program in the Big 12 South with OU/OSU clearly ahead of them and while Tech and Texas faded somewhat from where they were, we had Baylor ascending under Briles so there were still going to be a ton of road blocks from A&M ever sniffing a Big 12 title game in those years as well.