Usually those rollers are copper. I'm not sure that the would help a horse perform better necessarily, but they are designed to encourage salivating (which usually means a focused/relaxed horse)...some horses also just like to be able to play with the roller. Alot of roller bits, aren't necessarily a "high ported" bit either. Some of them are fairly low, still, even with a roller. I have one bit that has a straightish mouth, with rollers on it.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."