I believe the Spurs are going to be ready and primed for the playoffs. The Spurs have failed to beat the better teams this season because they haven't brought the necessary intensity and energy needed to do so - mentally and physically. As Pop stated, there is a smaller margin for error when you are an older and less athletic team. Once the real season starts, the players will raise their games with a sense of urgency.
I'm not a psychologist, I just believe its human nature to see a tough regular season match-up against a good team as just that - a regular season game. You can't really simulate a playoff game in the regular season because you realize there is a tomorrow and as Tony says "Meh, its the NBA. You win some, you lose some. Time to get ready for the next one". I'm not saying its right, I'm just saying that's the way it is.
We are all aware of how close the Spurs were to finishing off the Heat in 6 games on Miami's home court. When they failed to do that, they were forced to a game 7 for all the chips. How does that big game compare to playing a "hot" OKC or Portland team that has something to prove to themselves in the regular season?
You will see the Spurs players - from the Vets to the new guys - give everything they have in the playoffs. The mental mistakes and the lack of focus will be a thing of the past. If the newer players can't hack the playoff pressure (Jeff Ayers), they will be buried on the bench.