Yeah they might be better, but they're not $200 better than other name brand tires.
WHAT Kumho tires did you have on your Sonata? How many miles on the old tires? Did you buy this car new?
For example, their grand touring tire Primacy MXV4 vs the Kumho Ecsta LX Platinum. The Korean tire is probably 90% as good as Bibendum's for 60% the price. It's what youget when you want the MXV4 but can't afford them. Kumho seems to be among the best value-to-dollar ratios, particularly with the LX Platinum and their Ecsta 4X performance tire.
Getting into the name brand premium tires is when the diminishing returns start. It is very likely that Dunlop, Goodyear, Yokohama, Hankook, and Bridgestone tires probably would be just as good.
One more thing to consider: Basically ALL OE tires are garbage, designed primarily to be cheap