Emissivity and absorption coefficient are two sides of the same coin, you can't have one without the other (at a given wavelength) without violating the laws of thermodynamics.
OP, have you purchased commercial heatsinks that have performance data - ie. there is a link to that data ? Can you detail your thermal design and the Tj you are after for a given ambient? What options do you have for orientation of those heatsinks, and do you have data on the heatsink performance with orientation?
In this case I believe the OP is using these heat sinks on a sissysitR3 which also means he has 2 devices on each heatsink that has different thermal output. The mosfet is doing most of the work while the sit is coasting. I am also building sissysitR3 as well and want to get it right without buying more heatsink than I need between freight and material cost ( heatsink USA) it jumps pretty quickly.
Bill
Bill