Make sure your throwing with your entire body not just your specifically make you are throwing with your entire body and not just your arm.
1. To increase power first thing you should do is insure you're turning your hips before your shoulders rotate, if you have a dead base that is going toreally decrease the amount of speed you can get, by making shure your hips rotate first it stretches your core muscles and allows you to pull your shouldersaround to deliver the ball. Make sure you keep your body sideways to the target to maintain accuracy.
2. Make sure you get separation between your hips and your shoulders to further utilize your core oblique muscles, this is very important, this where a lot ofvelocity comes from the more seperation you can get the more you can velocity your body can generate. You should make sure your core is conditioned before youattempt this to avoid in pulls or strains.
Now your probabaly wondering what "hip and shoulder separation" means well look no further then the king of separation Tim Lincecum, you do you thinkhe can pull 98 MPH from his 5'7 frame? He uses every inch of his body and even Scott Kazmir who isn't a big dude back in the day pre TJ surgeory couldget up to 98.
The hips have rotated and there core oblique muscles are going to turn and deliver the ball, see how the hips are not in line with the shoulders but leaning?Power comes from the bottom-up, this is true even for an out fielder.
Derek Holland increased his velocity from 88-90 TO 92-95 and you can see how he utilizes his core here.
I don't really believe arm slot matters, Alex Rios goes from a low 3/4 and he has got one of the best arms in baseball, but remember if you change armslots your arm does not change, merley the tilt of your shoulders, the elbow should NEVER go above the plain of the shoulder.