It might be more likely than you think. If the startup and the founders are solid, and if there is a clear (and most importantly, fast) visa path like the one proposed, I don't think most investors would have a problem. If anything, $250,000 is way too low for a VC, so you might have to find an angel investor instead. Some countries would have an advantage over others, of course, but I don't think it's a showstopper - my first startup, based in Canada, received an offer for funding which would've put us over this threshold.

Even if the non-US startup will move to the US once they receive funding, the largely intangible advantages bestowed on a startup in the Valley (which I've witnessed in the month that I've lived here) give it a better chance.

Can you elaborate? What is the difference, once both startups are in the Valley, and assuming there are no language issues? So many startup founders are already originally from other states or even countries.