Not sure about your thinking on this. TCU was damn good prior to joining the Big 12. In 2010 TCU was in the MWC, played in the Rose Bowl (BCS) and beat B1G champ Wisconsin (P5 opponent). Had the CFP been in existance, they'd have probably beat Cincy as the first G5 school to make a playoff.
Outside the financial standing, and as far as how the TCU football program "fared" prior to joining the Big 12, they've really done about the same IMO. TCU's football program started getting "revitalized" in 1998 when Franchione took over. Hell, they beat USC in the Sun Bowl that year. While not a BCS Bowl, USC is damn sure a P5 blue blood.
Check out how their football since 1998.
TCU Horned Frogs Football Record By Year | College Football at Sports-Reference.com