I have seen pro racers with their brakes mounted that high on the bar, so that position works for some riders. But if it doesn't work for you, you should move them. I would not go more than 1cm farther down on the bar.
Moving the levers lower will make it easier to reach them from the drops. It gives you a little less leverage from the hoods, but the change is minor.
You may also prefer a different bend of bar that has a larger radius curve. That leaves more room for larger hands.
If your front brake works "pretty good" but your back doesn't, then the problem isn't lever position. Check the brake's pad alignment, and the rear brake cable.
If you move the levers down on the bars you will need to remove the wrap, move the brake cables and re-wrap the bars. You can't just slide the levers down on the bar, that's dangerous.