Power development for batting performance can be improved drastically through proper strength and power training. It is not to be trained through these ridiculous imitations of sport specific movement such as adding resistance bands or tubing to your bat and taking swings against the resistance of these tubes. I’ve also seen implements being attached to cable systems while mimicking swing patterns as well.
Every hitter is entitled to their own style or preference when it comes to stance, set-up, and load. However, when the stride foots lands, all hitters are very much alike in their movements to and through contact. My emphasis will focus on the “non-negotiable” of consistent, hard contact—bat path. Learning to control the bat barrel is an enormous step forward in becoming the best hitter they can be.  
I'm an agnostic when it comes to a silver bullet statistic. Sure, it might exist somewhere, but I haven't seen evidence to support that. Instead, I think the sheer number and variety of statistics we have show just how much beauty and nuance there is in the game, and why it's the greatest game that's ever been. That said, I like OPS+ because it incorporates many different things -- getting on base (i.e. not making outs) and slugging, while also putting those performances into the context of a time (the season) and place (ballaparks). I also like the simplicity of 100 being average -- so if you see something like Barry Bonds' run of four years with a 230 or better OPS+ is simply superhuman (or some may say, unnatural).
Keep your hands in. Whether you are dealing with an inside swing or an outside swing, it is important to keep your hands close to your body. Most of your swing is generated in your hands and wrists. If your hands are extended your bat speed will slow and your power will drop. The palm of your dominant (top) hand should remain facing upwards through contact in order to drive through the ball.
In cricket, a player's batting average is the total number of runs they have scored divided by the number of times they have been out. Since the number of runs a player scores and how often they get out are primarily measures of their own playing ability, and largely independent of their teammates, batting average is a good metric for an individual player's skill as a batter.