Given that there are a million people admitted yearly for CHF exacerbations, and 500,00 acute MI patients, an increase in mortality of a mere 0.5% translates to 7,500 dead people. That's a lot of coffins in my opinion.
The difference in quality is very likely due not to additional time in practice, as the longer a physician had been out of medical school, the more likely their patients were to die. On the one hand, that is surprising because we would tend to believe that experience and continuing education would reduce mortality. But on the other, it seems that no amount of continuing education and experience can make up for the substantially improved amount of knowledge contained in an up-to-date medical school curriculum as compared to one from decades past. The more likely reason that they outperform US grads is that to even make it into a residency in the US they must be a superstar back home, then perform exceptionally on a test that isn't even in their primary language, then complete what is often a second residency. So our FMGs are, more often than not, world-class physicians that we acquire via their filtration by multiple training and certification processes. Losing them would certainly be a blow to our medical system, as they make up a disproportionate amount of our medical school educators, researchers, and the physicians that care for the underserved, in addition to their already excellent clinical skills.