If my memory of high school physics is right, the strength of diamond comes from its more complex interlacing latice structure. Counter to intuitive reasoning, you want pockets, tiny holes, in your heat conductive surface as opposed to a solid mass - this is of course speaking at the microscopic level. Carbon is known for its varying complex latice structure of its bonds, diamond having one of the strongest.
I'm a little surprised more hasn't been done with graphene in the past. Carbon has been worked into everything, you would think there would have been a push for something like this a while ago. I'm more suprised that composite metals haven't been pushed more since they are usually cheaper than any pure metla alternateive, and often can be made at equal or better performace standards.
My PC awaits the replacement of my copper CPU and GPU waterblock with a carbon/copper composite one!