I run in crossfire and get

14,300 with the cards @ Pro and Q6600 @ 3.0 on a X38. You aren't using Rivatuner to adjust the clock speeds still are you as only one card overclocks.

With them both @ XT bios and Q6600 @ 3.5 I 17,500.

BTW the x16 x4 ports are meant to give it a 10% hit