I'd still go with Foodland (Sack-n-Save)/KTA/Choicemart.
1) The poke is head-and-shoulders better -- and considerably more variety
2) Costco doesn't sell Kona Chips or One Ton chips.
3) Lum's Char Siu sauce. Costco doesn't sell it. The local markets do. Sadly, there is no option other than "buy way too much" because nobody can use 11oz of Char Siu in a week or two. (Pro tip, some markets sell pre-marinated pork and chicken with Lum's.) And just a heads up -- anyone who finds themselves with leftover Lum's is welcome to drop it off at the farm. I'll put it to good use.
4) Dragonfruit, Passionfruit, local avocados, local citrus, local tomatoes and especially local onions. It's even better to buy these at farmer's markets (when possible). But the local supermarkets typically sell this and Costco does not. It chaps my posterior that Costco sells California avocados when I let 10,000 pounds of them rot on the ground every year for want of better options.
5) Coffee. Costco sells two Hawaiian varieties -- the two biggest producers. The smaller markets have much better options.
6) Finally, Costco sells mostly US stuff. There's nothing wrong with that. I like maple syrup as much as the next guy (probably more than the next guy). I buy brisket and make pastrami. I truly like Costco. That being said, the local markets have far more Asian foodstuffs -- everything you need to build a curry that would bring a tear to the eye of an auntie.
The big-picture problem with tourists shopping at Costco in Hawaii is that they tend to go there first, buy 10 days worth of food (for a one-week trip), and then that's that for food -- they're not going to buy from the smaller producers because they loaded up on corn dogs and Eggo waffles. I go there every couple weeks and that's what I always see -- people buying all the same stuff they can get at home.