iv 75+-, worth to power up?

Magicarp: I would wait for a 90%+. You have collected so many carps, you will want a real high-cp-gyarados. Hard to believe, that you do not have a higher-iv-carp already.

Snorlax: Great, good IV, best moveset - i would go for it.

I wouldn't power up anything under 80 at the minimum, preferably 85-87+ depending on the Pokemon/moveset/rarity/etc.

While some may be debatable, I especially wouldn't power up a 76 IV Gyarados, given the fact that karp is a common 2km egg hatch, and you have to catch 100 of them for 1 evo. Odds are you'll catch or hatch a better one right after you pull the trigger. Aim for 90+ for him. It's just a numbers game

However, Snorlax/Lapras can be the exception, given their extreme rarity (could go 6 months without seeing another)

If they would help you out, and you're just sitting on the candy letting it waste away, go for it. YOLO. But Snorlax has a defensive moveset so make sure you're got the candies to take him to lvl 30 so he can actually hang as a defender. Lapras, given CP nerf, is primarily offensive now. I wouldn't hesitate to power him up if you need help w/ Dragonites.

Snorlax is a no-brainer with these IV and that moveset.

Lapras is a no from my side, unless it has 15-15-3, since HP is the least important stat with the high base and you want the highest possible attack IV. I hatched a 76% Is/Ib one about a month ago and didnt power it up yet. Lapras low max CP require a high trainer level to make up for mediocre IV. So you should be at least 35 or 36 I would say, it depends on your area though. But with less than ~2650 CP (that equals a Lvl 30, ~80% Vaporeon), it would be really hard to find a spot in the gyms in my neighbourhood.

I had a level 30, 75% IV Magikarp (with perfect attack stat) and I decided to evolve it - I got a bite/dragon pulse Gyarados. I don't have any regrets at all. After all, it took 6 months to get enough candies (including over 100 km of walking); I see maybe 1 Magikarp a week. Who knows how long I'd need to wait to get a better karp? And besides, even with those "low" IVs, my Gyarados still sits above all of the Snorlaxes and Vaporeons in gyms.

What are the exact stat IV's of the Snorlax? Only then will you know if it's worth powering up. I wouldn't worry about Snorlax's stamina IV, given how high its stamina already is.