As someone noted above, its important to distinguish between the box +/-, which is what shows up in the box score, and "real +/-", which adjusts for the quality of opponents on the floor and teammates on the floor. BPM can be misleading because you can run up a good point differential against poor opponents, or get carried along by teammates who are having a good game.
I've never seen RPM stats calculated on a game by game basis, but from what I've read they are best used as a season long average. RPM used this way seems to me the most important stat out there.
None of this is meant to claim that Smart did or didn't have a good game. Best way to do that is to watch the tape.