Hi hoagy1969,
While I don’t have personal experience with the Sleep Science Emma mattress (that comes with the adjustable base) .,. you can see my thoughts about this combination in post #2 here.
The adjustable base is good value for a split king but I personally wouldn’t consider the mattress because the comfort layers are very low quality (3 lb memory foam and a 2" air comfort layer that is 1.5 lb density for a total of 5" of lower quality foam that could be subject to early softening and failure).
Buying a mattress from Costco is a roll of the dice at the best of times in terms of finding one that is suitable for your needs and preferences because they have little knowledge or ability to help you make a suitable choice but at least if you don’t choose well they have a good refund policy. In this case though you would be rolling the dice twice. Once to see if you have purchased a mattress that “works” for you in the short term and then again because you would be buying a mattress that uses low quality materials that may soften and degrade, lose its comfort and/or support, and no longer be suitable for you after a year or two (and this isn’t covered by warranty).
I also wouldn’t buy a Chinese made mattress that didn’t disclose some kind of meaningful certification for offgassing and harmful chemicals in the mattress and I also wouldn’t buy a mattress that had more than an inch or so of 3 lb memory foam.
Phoenix