Thing is, Mantle is to reduce CPU overhead in CPU bound situations. So as enthusiasts, who OC their CPUs, we'll notice much less performance gain than your average user running bone stock in the configurations that they are running in the article. So a situation that might be in the article where they are running a bone dead stock 3970X with two 290Xs, would be MUCH less of a performance improvement than their 55% (although it could scale given both components, CPU and GPU are OC'd at the same time).
I think the greatest effect Mantle will have is on a core/AMD-CPU limited high-end Crossfire setup that isn't OC'd.
Still, impressive nonetheless. Any advancement in tech is always a solid thing to see.