FCP itself can only address 2 cores.
When exporting, you can use Compressor which can use all 8 by using a virtual cluster. But for this to work, you export a reference movie of your timeline and bring that into Compressor (you can't simply Export via Compressor).
If you run a search in the Mac Pro forum, there is a thread about creating clusters in Qmaster. Creative Cow also had a podcast entry recently that specifically addressed this.