"In the 1940s, should the US decide to go bananas (in and of itself ASB), the world would be hard-pressed to stop it."
Ah, no, I think you are wrong here Mike and are forgetting the problem of numbers. In this ASB Nazi America v the world scenario, the US will have to conquer the world, or at least large portions of it as bases and bulwarks to keep the rest under control, and that type of garrison duty takes more manpower than any US state will have. The USN may well be able to control the world's seas and still have the resources for an air force able to take on the rest of the world. Note that the US didn't have to do this in OTL, US airpower by itself IOTL was not more powerful than the RAF + Red Airforce + Japan + Germany. It's possible it could happen, but the men, money and resources will have to come from somewhere. What branch of service or the economy suffers as a result?
Even with Nazi-levels of conscription, the US doesn't have the population to support a navy, air force and army to beat the world or the economy to support those efforts.
The next problem is the US army. IOTL it wasn't very impressive. The US army didn't face anything like the Eastern front, or even the variety of challenges the British did from 1939-1945. In this scenario they will have to face such problems on a regular basis. Operationally the US army IOTL didn't perform that well, they'll need to improve dramatically to keep the casualty levels acceptable. Unless you have a radically differently trained and equipped US Nazi army to that of OTL, it will get bled and then shredded by the Germans, the Russians, probably the British too. Is the ASB Nazi America going to be equipped with wonder weapons or those of OTL? If it's contemporary American equipment from our WW2 then they're going to get walloped against quite a few opponents. Also, making the US soldiers and generals Nazis isn't likely to make them any more competent. If anything, fascist regimes demonstrate remarkable levels of incompetence in important areas because they allow ideology to over-ride reality and common sense.
I suggest that in this scenario there will be no US nuclear weapons, or they will be very late in appearing. It's not just the absence of Jewish scientists, most scientists won't want to work for such a programme. The start of an effort to develop a Nazi American bomb will be greatly delayed (I can't see Albert Einstein writing to the Nazis in any country to raise awareness of the possibilities of the A-bomb), the suspicion of 'Jewish science' will still be there as it's ideologically driven, foreign scientists will not be journeying to the US as they did IOTL, the UK won't be helping the Americans get going as they did IOTL. There's a greater chance that the UK, USSR and/or Germany will develop the bomb first in this scenario.
The other thing to consider is what state the US economy will be in? As the OP talks about 'Nazi America' then we have to assume that the political and economic management would be similar to that of OTL's Nazi Germany. In brief, a madhouse of internally competing interests generating waste and a succession of changing mad plans. US industrial efficiency, and in the long term industrial might, will be lower than that of OTL. The question is how much?
Conquering Canada and islands in the Atlantic and Pacific would be fairly straightforward, but that's completely different to trying to conquer Europe or China. The US just doesn't have the manpower to do it, not even a Nazi America that sends every able bodied male to the front. A Nazi US will be able to dominate the western hemisphere because of weight of numbers and economic might, but invading the other continents is a very different challenge.
The OP also suggests that it's an America v everyone else scenario. In the 1940s the 'everyone else' is too strong.