my PhenomII X3 gets ~7800+khash/s already on jgarziks cpu-4way, or ultrasofts cpu-miner,
an X6 should get about twice as much.
I'm running an X6 1090T BE, not overclocked and can confirm that I get ~17500 khash/s.
Are you running with the 4way switch? I have the same proc, and I'm getting ~20khash/s with pop's cpu miner.