I use a 2.8 bat, I'm not much of a puller, but the cuts one of my major shots. Any heavier and I wouldnt hit the ball nearly as hard, I need a light bat to generate enough bat speed. Did you feel like your hands were getting through the ball quicker? Also with a heavier bat you could have been thinking about what it feels like using a different bat, resulting in your mind being clearly as the balls fed into the machine, so you play the ball better (I doubt I explained that very well). I know I usually play better, espcially on the machine, when I'm thinking of one specific thing, the rest of my technique will just take care of itself after that.