Power up jolteon?

Anything over 2000 CP might as well keep the same. You can kill any Gyarados with it.

I think it's worth powering up, but unnecessary given how easy attacking is.

But then again it's only 8 candies and 10k dust to get it to 30, so you could say "you may as well" if you want to shave a second off each Gyarados battle.

Power to lvl 30 yes, definately worth. My lvl 25 with Thunder can beat more than 1 vaporeon if i dodge, pretty useful, so go for it.

He's certainly worth powering up, but the question is how much priority to give him? The answer to that largely depends on a number of factors, including how common are high level Gyarados in your area, how strong is the rest of your attack squad, and how strong is your defense squad?

I really like using my Jolteons and think yours is worth the candy, A good attacker against Gyarados, Vaperon, Lapras, Dragonite...