[cesarb]

> All I know is that you'll need higher amperage with lower voltage, but it can compensate proportionally.

This discussion is assuming that the amperage is the same. The common "default" AC socket in the USA is AFAIK the NEMA 5-15, which is a 15A socket; the common "default" AC socket in other countries is AFAIK usually either 16A, 15A, or 10A, so at most you'd have one extra ampere.

Of course, if you're adding a dedicated socket where you'd expect to plug a car (or other high-power devices like a large air conditioner), you'd put a higher-power socket like a 20A one. But this discussion is, as far as I understand, about what you could find on a random garage; I believe it would be unexpected to not have at least one common "default" AC socket on am enclosed garage, but finding a higher-power AC socket would be less likely.

[justinclift]

Any idea how common or feasible it would be in the US to get ~400v 3 phase power to the garage?

Here in Australia it's possible, though not very common. ~400v is useful for some larger sized machinery.