I don't think the U.S.A. was "much better" at developing weapons than anyone else. Our airplanes were good, at least the post-1942 versions were, but so were everyone else's. Anyone who says the Germans didn't develop good warplanes never got into a fight with one (or more) of them. British aircraft and pilots were renowned for having a go at anything flying. The actual airframe performance of the Spitfire (and others) was always outstanding. The Japanese pilots and airplanes were excellent, at least early-on in the war, before they lost a lot of veterans and failed to update their front-line equipment in greater numbers. The same can be said of most any air force that got to fight enough to learn aerial combat tactics and employ them correctly. The Soviet Union took a little longer just due to outdated aircraft at first and inability to let their pilots think for themselves. In the end, Soviet pilots and aircraft were right up there with anyone else.
We got a good one in the XP-72, but it came right when a better airplanes was not particularly needed as the war was winding down, so it never got adopted. Had it been adopted, it would have shown sparkling performance at the cost of heavy maintenance. Have you ever tried to change 56 spark plugs? Just getting an R-4360 started after it sits for a week or two is several hours of oily labor to drain the bottom cylinders. You'd likely have to remove 18 - 20 spark plugs from the bottom cylinders and turn it over by hand to clear the oil accumulation.
But, when it is running right, it CAN and DID give pretty decent performance. The numbers I showed above came from a quick online lookup of performance, not actual flight test reports. I don't think I've ever seen an actual flight test report on the XP-72. But, when we want to compare, we use the numbers we can find.
Do you have information that the numbers above are incorrect? If so, post it, by all means. I'd love to see flight test reports showing the performance seen during testing rather then reported in reference books with backup data cited but not shown.