When you do any kind of training, there is something called "specificity of training" you have to keep in mind. Specificity of training means that your muscles will be most effective if you use them the way you train them, hence you should train them the way you want to use them. This isn't the absolute rule depending on what you're looking for, as varying your training by repetition speed is also a good way to break through strength plateaus.
Actually, going with slower repetitions is a great way to make a lower weight hit your muscles harder. Not only that, but it also makes sure your muscles are getting strong equally all throughout the movement range, rather than just in the position where you're doing the push that generates the momentum that gets the weight to where it ends up. Usually, if you're "throwing" weights, relying on momentum, you're probably not bringing the weights back slowly either, which makes the previous statement all the more true. Again, if you're looking to do fast, hard punching, then speed is what you're looking for. Just make sure you're not relying on momentum to get it done; make sure your muscles are working equally hard from beginning to end of each repetition.
However, what you're doing is more like isometric training. You are generating more strength work in the areas you pause in than anywhere else. This is a more extreme version of what I stated above. Pausing at the down position would be most useful to generate a lot of power and speed for a punch. Pausing at the up position wouldn't do you so much good. However, this pausing isn't going to do you much good at all for endurance; it's the number of repetitions you do to the point of muscle exhaustion or failure that makes the difference.
For muscular endurance with a bit of strength mixed in, go with 12-16 reps to failure. For pure endurance, go for 16-20 reps. Muscular endurance makes a given workload less exhausting when done repetitively, like punching or wrestling. Be sure to include movements that you will be using to make it specific. Push the weights like you would punch or push.