I had Nankang on a Z4 we had for a short period, as they came with the car. After my wife came home telling me she nearly lost control in the wet. I took the car out and found out exactly what she meant. In the dry the seem OK, defiantly no premium tyre but serve a purpose. In the wet they were awful, changed to Falkens within about 3 days of those events. Now have 4 Nankangs sitting in a shed.
Vreds and Falkens should be the minimum you look at.
In terms of Nankang vs Kumho, i driven in friends cars with Kumhos and have to say they have been fine in all weathers. I can only go by my 1 experience with Nankangs; which was bad