At least in NETGEAR's case, the products are physically the same. Only a tweak in the firmware to lock out certain MCS rates.I've never had a Costco membership, but past purchases from customers have led me to believe that Costco, BestBuy, etc. offer worse products (from computers to routers and more) than what the manufacturer offers in general. 'Worse' may be a bit harsh here.
But I wouldn't want a crippled product (no matter how slightly crippled it is) to save a few pennies at most.