Empirically, opt-in vs opt-out doesn't matter. Kieran Healy writes about this extensively. I think he discusses other things that actually have resulted in large changes in Spain and Italy in this paper. This time series is nice.

One relevant fact is the in virtually all jurisdictions, the heirs can and usually do veto donation. [Edit: Actually, just one of many obstacles, probably a small effect. I forget why I included it.]

In the United States it's kind of neither. When you get an id card there is a yes/no checkbox you need to check.

It already is in Italy (and I'd guess in much of the rest of the EU too).