What if Pius XII had spoken out?

At the time, in the 40s, people had a firmer grasp of what was possible. Father Rutler cites The Tablet.
In Belgium at the start of 1943, the Germans would not let Cardinal van Roey publish the Pope’s Silver Jubilee address, and the Italian government banned the film Pastor Angelicus about the life of the Pope. In that same January, the London Tablet commented on the tendency to think that more would have been accomplished by a louder protest from more bishops: "If there exists a vague atavistic memory that once Popes and Bishops spoke, and wicked Kings trembled, that salutary thing happened because the public opinion of the day had a much fuller and deeper sense of the rights and importance of spiritual authority.  Modern men, who have for so long applauded the narrowing down and emptying of that authority as the emancipation of mankind from the thralldom of superstitions, can hardly be surprised if, as a rule, prelates in the modern era tend in prudence to limit themselves to the field indubitably conceded to them by public opinion."
Be careful what you wish for, you might get it. Rutler also gives evidence of Pius' actual policy.
In a letter to Bishop von Preysing on April 30, 1943, Pius XII described with unusual candor the theory behind his subtlety "We give to the pastors who are working on the local level the duty of determining if and to what degree the danger of reprisal and of various forms of oppression occasioned by episcopal declarations…seem to advise caution. Here lies one of the reasons, why We impose self-restraint on Ourselves in our speeches…The Holy See has done whatever was in its power, with charitable, financial and moral assistance." The U.S. diplomat Harold Tittman recorded how anti-Nazi resistance leaders consistently had urged the Pope to follow this policy.
On at least one occasion Catholic Bishops in occupied Europe spoke out against the actions of the Nazi government. This was in the Netherlands and the result was the rounding up of Catholic Jews. On 26th July 1942, the Dutch Bishops had a letter read in all Churches denouncing Nazi policy. On 2nd August, two weeks later, the Nazis began rounding up Catholic Jews, including Edith Stein. By 9th August they were in Auschwitz and by 30th September most of them were dead. When they were arrested they "were told that they were rounded up in direct retaliation of the condemnation of the Nazi "final solution" by the Dutch bishops".

But some would rather be hoodwinked by the KGB.