If you need high performance, I'd recommend working with a supplier specifically in the heat sink market. I vaguely remember, from many years back, finding the thermal resistance of various alloys and extrusion methods has a wide spread, and you don't want to find you have the wrong one. Treating the surface where the heat source clamps on can also change performance a lot. You can get the oxide stripped, and a combination of platings applied to end up with a copper surface (you can't plate copper directly onto aluminium. I assume there are multiple layers in a CCA cable, too). Its takes only a surprisingly thin coating of copper to help spread the heat info the heat sink, and reduce hot spotting.