Thread are threads. The more they are the more they are split between multiple cores when necessary by the OS.
Try assigning a single core when you are doing some serious stuff or even standing near crowded aetherytes and see if you don't suffer a performance loss.
And no, percentage doesnt show cores usage. I can have a 16% usage and spread the load between 8 cores.

Replying to OP, I've a 780 and go down to 30FPS in some cases even with 80% of load. It looks more like a game limit.