I was under the impression that the speakers were the ultimate arbiters of how much wattage was passed to them. That is, the speakers might never ever use more than 30W of power, and, therefore, slapping a 400W amp back there wouldn't do any worse or better than a 30W amp (ignoring quality differences between the amps).

For the most part, you are correct. Just because you have cheap speakers doesn't mean you need to use a cheap amp. In fact, a better amplifier can tend to make even factory speakers sound better.

If you intend to upgrade the factory speakers to nicer ones, it might be a good idea to get a powerful amp now. Just turn down its gain controls low so that you don't blow the factory speakers.

But if you don't intend to upgrade the speakers, then there's no sense in wasting the money on a beefy amp.