Worth or Not ?

I can't help much without mons lv.
If you have another good dark attacker and you like to dodge I would go for alakazam.
If you don't and your area gyms are filled with exeggutor and slowbro I would go for gyarados.

if you suffered a lot to evolve these 2, you should power them up to lvl 30. If you usually find a lot of magikarp / abra in your area, i think you should just farm better ones.

If CP are not very low, I would power them up ;)

Low IV Pokemon can still be effective with useful moves imo.

My 6/15/3 53% WG/HP Vap has been very effective on attack (obviously with 15 and good moves) since I've had it since I started playing in August. I had no idea of IVs when I first caught that Vap so that's why I noobishly powered it up. But it's been great on attack and still is.
I stopped powering it up a few weeks ago before the CP buff because I felt it's not going to make much difference and I'd rather keep the dust.

I caught a Gyrados with the same moves in my driveway yesterday, 2252 CP low IV. I read Dragon Pulse is kinda crappy but I'm not sure. I won't power it up and my 91% 15/14/12 Gyrados with Bite/HydroPump I stopped powering up after the CP buff because 1) it's fine as it is (was 2100+, buffed to 2500+), 2) I destroy 3000 CP Gyrados defenders with electric.

I'm glad I stopped powering up certain pokemon that I was keeping maxed out. I learned a lot of them aren't going to get much better. At least I don't think so.

The Alakazam is a great attacker with that moveset. I keep getting Confusion as a quick move with my Abra evolutions. I'd take 78% IV any day. If you've got the stardust and don't think you'll catch a higher level Abra to evolve anytime soon, I'd power it up.

The Gyarados will reach insane high CP with that IV and also has the best defensive moveset if that's worth anything to you. Since he is 98% I think you won't regret powering him up either.