If you look at Curitiba (the Brazillian WTCC track) it is a track very much suited for high speed. SEAT were compromised on tyre wear so may of had the car setup to deal with the increased temperatures and increased wear associated with higher speeds and higher braking speeds too. This had the effect of giving them better speed on the straight bits, but harmed the cornering speeds as they were trying to conserve the tyres.
It was historically a BMW track, so the fact the SEAT's bucked the anticipated trend at Curitiba winning both races was very good. It may well be seen that they suffer at other circuits, we shall have to see.
For example at Thruxton the BTCC run all the cars with a special compound of Dunlops to cope with the high speed wear rates associated with a track like that. I'm not sure if WTCC do the same.
Many of the UK circuits are less of a drag strip and much more twisty where slower speeds are maintained.