Power up Ampharos?

Depends on what you have. The legendaries are the ones most would prioritize first.

If you have a good IV one with right moveset and resources to back it up - you could power him up.
He could be good for some future legendaries like Kyogre or even Ho-oh as optional attacker.

It's a bit of a strech but considering mega evolution and if that will (hopefully) someday get implemented in Pokemon Go he could become an ultimate water-type counter thanks to dragon typing.

I maxed out mine but regretted it. With Raikou and Zapdos around, it is more of a trophy now.

I'll pass. I'd rather spend the dust on a stroger electric mon e.g. a Raikou plus I'd rather walk a Tyranitar or a Dragonite than a Mareep linage for the candy AND I'd rather use the rare candies for something more rarer like a 20 km Legendary buddy.

Probably not. Use that same dust on a Raikou, Zapdos, or Jolteon. They're easier to get and more useful.

The only advantage it had over Jolteon before legendaries were added was that it was bulkier and the only bulky electric, and even then it was a stretch to power it up because of it's moveset.

With Raikou and Zapdos in the game there's no reason to power an Ampharos for battle

Raikou's got better stats and moveset with the same typing, so there's literally no situation where Ampharos is better than it.

Zapdos has better stats and flying typing which might have potential use in the future, as well as access to Thunderbolt which is better than Zap Cannon.