Really? I suppose it depends on how you score it.
According to Wikipedia (I know, biased and anti-Catholic. Work with me.) In the 1920s there were about 3 million Jews in Catholic Poland. In 1930, there were about 15 million Jews worldwide, of whom about 4 million lived in the US.
After WWII, there were virtually no Jews left in Poland. The world population of 15 million had been reduced by 6 million; in other words, about 40 percent of all the Jews in the world were murdered. If you take into account the fact that 4 million were in the US and therefore out of Hitler’s reach, you could reasonably say that Hitler murdered 6 million out of the 11 million Jews he could reach, or about 55 percent. (Keep in mind also that many Jews fled Germany for the US or UK or, in a tragic exercise of bad judgment, Poland, after 1930 but before the war. Others lived in places like the Soviet Union – about 2 million there – and therefore may not have been vulnerable to Nazi occupation, but let’s not quibble.)
We can argue all we want about what Pius XII and St. Maximillian Kolbe believed and did. We can argue all we want about the role of Catholic Germans, Poles, and Italians in the Holocaust. Maybe they did the best they could; maybe they didn’t.
But to characterize their efforts as “effective” is laughable.