Hi FlausFSU,
You can see the guidelines I would use for a memory foam mattress in post #10 here. For me CertiPur certification would not be as important if the foams were manufactured in North America (and most American foam manufacturers are CertiPur certified anyway) but if the foam was manufactured overseas it would be an important part of my purchase decision to make sure that the foams have some reasonable expectation of being “safe” in terms of harmful substances or VOC’s.
Classic Sleep 11" Gel memory foam:
This mattress uses low (3 lb) to medium (4 lb) quality/density materials in the top 3" and “just barely” meets the minimum standard I would use for a memory foam mattress (no more than an inch or so of lower quality materials). They are also CertiPur certified*. 4 lb memory foam can make a good choice as part of a mattress design (and Tempurpedic uses this density as well) but around 4 lb memory foam (+/- a little) is the lowest I would go and in combination with 3 lb memory foam I would tend to avoid it even though the 3 lb memory foam is only an inch. It would also not be a good match to the Cloud Supreme and would be firmer. The listed price is also outrageous for a mattress of this quality.
ADMIN NOTE: *Removed 404 link|Archived Footprint: certipur.us/pages/for-consumers/find-products/
This uses much better quality memory foam in the comfort layers (which are the weak link of most mattresses) and as you mentioned it is a much more realistic comparison to the feel and softness level of the Tempurpedic Cloud Luxe. The base foam is 1.5 lbs. this is much better quality and value and among the better budget memory foam choices available.
This mattress includes 2" of 3 lb memory foam which would exclude it from consideration. Sleep Science is also manufactured in China and isn’t CertiPur certified so that would be another reason I would exclude it. Costco customer service has little knowledge of mattresses and I doubt that you could get any meaningful information about which of the Tempurpedic mattresses this may approximate and as you can see in post #4 here … buying a mattress from Costco is really a roll of the dice when it comes to it’s suitability or the knowledge that is available to help guide you and the only good thing about a Costco mattress purchase is that you get a free roll if the mattress isn’t a suitable choice. I would avoid this mattress in any case no matter who was selling it.
Out of these three there are only two that meet the guidelines and of these two the Classic Sleep is “on the edge” in terms of quality and is not good value at all at the listed price (or even substantially lower).
So making final choices is usually a matter of eliminating the worst options and then choosing between “good and good” but in this case I would exclude two of your three options anyway so a final choice would be easy between these three 
Phoenix