I went from flat to angled and believe it is an improvement.
Did not make any reproducable tests, but I think there are test results with people who get better aerodynamics.
More important is that it easier for me to sink the head between my shoulders. I can reproduce that when I’m sitting at a desk: during the angling of the underarms upwardly, my head falls more between the shoulders. Maybe you would expect the opposite, but it seems that the muscles and tendons are structured like that.