efficiently. On the westbound leg, the jets were typically going out with limited cargo and passenger loads on the legacy A330s. SLC wasn't enough for Delta's 777s and they were used elsewhere, and I suspect it would be tough to fill an A350 as well. That leaves the A330-900neo, which I still don't think has the legs to do it.
KAL could do ICN-SLC-ICN easily with the 787-9 though, and they have plenty of them to support the route, not to mention that KAL is a major SkyTeam partner.