Topic says it all.

Setting
sys_spec_PostProcessing
to 4 in autoexec doesn't allow Crossfire card usage to go above 50%. Crossfire scales fine if this is not in config file.
Not sure about SLI.

Still haven't found a workaround for this.

UPDATE: Its actually PostMSAA that is the culprit. If its enabled, Crossfire bombs. Please check top see if this is the case if you are using a multi-card setup.