Is it worth to power up a Cloyster now ?

Def-Buff should compensate low HP. Effective health is the product of HP and Def, so Low HP high Def is from defensive point of view the same as High HP and Low Def. However, there are certain aspects (of moderate relevance) to consider:
(1) Low HP means, it needs less potions to heal (positive for cloyster)
(2) Each loss of HP charges the Energy of charged attacks. This mechanic is less effective for a Pokemon with less HP. (negative for cloyster)

For me, there is no question, I now get the chance to build a dragon-slayer, until now i did not find a lapras.

If you don't have a Lapras for sure. Since I have a Lapras I'm enjoying all the towers full of Dragonites. The ice attack kills him very fast and it's easy to dodge him.

I would not power that up. Rather that you evolve high CP Cloyster when you hit level 30 and also use Goldduck, Slowbro and Blastoise with Ice Beam as temporary fixes. I just dont see how you would benefit from a 90% low CP + massive stardust vs a low IV high CP mon with less 300 cp at level 30. The main difference will be that you will use maybe 20% more potions which is an acceptable tradeoff since you will be able to use that stardust elsewhere. I think with this new meta stardust is so key since the best def movesests mons have lower CP thus becoming much more expensive to build.