Info on Ceika 1M Coilovers?

It's hard to say anything definitive. Ceika coilovers have one main advantage: every set is pretty much a custom order. You can ask for valving of your choice and mix and match the other components. How well they do the valving and how different the options really are from one another is hard to say. Component quality wise I don't see why they would be worse than others at the same price point (including Powertrix and Stance).

If you're in North America, you can probably get a set from Powertrix or Stance for the same price, including shipping. I've read that with Stance you can also get their other types that are not listed for the Z31 but can be made to order. If you're looking for the kind of setups they offer, you can rely on the recommendations from all the others who've used them. If you don't plan to take your car to the track and would like a ride that's still on the comfortable side, I'm not sure that their setups are the best fit. That's the impression I've got from the different experiences I've read about.

Reading online about people's experiences with Ceika coilovers, you can see that they often get some component wrong the first time around, just like in the video you linked. If you go with them, you have to consider this possibility and be ready to wait for replacement parts to be manufactured and sent out. Delivering on time is not their strong suit.

It's pretty unlikely you will get a comparison from someone who's run either Stance or Powertrix and Ceika coilovers.

Two main reasons why I did not seriously consider Ceika coilovers, valving integrity and spring quality although there is probably a delete springs option available so you can use local springs instead. Then it's all very well for someone to claim that they can valve to your requirements but you are relying on their expertise in that area which is notoriously questionable for any SE Asia supplier. Another reason is that I'd prefer to buy from a supplier who specialises in suspension parts at a consumer level, that way you are more likely to get a proven product that will work well out of the box for your car.

Completely bolt-on? Do they cast their own spindles or do you still need to cut/weld the oem strut tubes? Seems unlikely…

"you will receive a complete bolt-on suspension system with the exact same bolting design/pattern as OEM."

Installation is the same cut/weld strut tube design as other suppliers. The only true "bolt-on" coil-overs I found were in Japan and they cost almost $5k!