Of course the TD should have been called back - and therefore the missed extra point would have been gone too. The way Texas was raming the ball down USC's throat at the time, they would surely have scored on the next few plays and probably would have nailed the extra point. So, very likely the bum call gave USC a one point advantage. However, it was of zero consequence, as Texas overcame a 12 point deficit to win the game.
"The way Texas was raming the ball down USC's throat at the time, they would surely have scored on the next few plays and probably would have nailed the extra point." You don't know that. They could have, at best, wound up with a FG, the same way SC stopped them in the previous drive. And since he just missed the extra point it is just as likely he would have missed a FG. We'll never know. Agreed, though, the way Texas came back from 12 points down was impressive.
Also take away USC's field goal at the very end of the half. Even if USC had held Texas to a field goal rather than a touchdown that wipes out the missed extra point and also the time needed for USC to get a FG at the end of the game - a 4 point swing.
If the refs had made the correct call, Texas most likely would have scored a TD anyway and not rushed the extra point, USC most likely would not have had time for a last second FG so USC gained 4 pts with the call.
The most likely scenarios either give Texas another 4 points or makes it a wash.