You can cache animation names / sprite names -> ids if you'd like to save the lookup. Calling strings frequently isn't going to be as fast as that, but as with everything else it depends on what else you're doing at the time.
If you want to you can do this like so:
int animationId = animatedSprite.GetClipIdByName("...");
and call
animatedSprite.Play(animationId);
later.
In 1.90 final, you willalso be able to do something like this
tk2dSpriteAnimationClip clip = animatedSprite.GetClipByName("...");
and call play with this.
The advantage to this is that your clips could be from different animation libraries.
Once again though, this is only necessary if it is becoming a bottleneck..