I think my thumbs have gotten the muscle memory down of hitting Left/Right + X fast enough that it's what I always use. I don't macro anything on a controller. Granted, if I'm doing this while running, I do this goofy "put my right hand almost horizontally over the controller body in order to target and run at the same time" technique :P Also, I normally pre-target when the opportunity is available. For example, the mob I'm currently on is very close to death. Before it die's, I start targeting the next mob using Left/Right on D-pad.

All in all, I am probably not that efficient, but I also think it's fun this way, and we're usually talking split-second delays comparatively. I also use a 360 controller, so not sure how much a difference that would make compared to PS3 (I imagine it's about the same).