mnm99,
You may be looking at the wrong piece of the puzzle. The heat sink is one piece, the component is another.
If the heat sink gets "hot" it is doing its job; how well, that is the question. How "cold" can the heat sink keep the component?
Another related question, how "hot" does the manufacturer say the component can get? If you haven't already, learn how to best use your IR gun, then measure how "hot" the component gets.
For example, I've seen "warm" heat sinks where their associated components got so "hot", they effectively de-soldered themselves.
And one more point, don't forget about electrical isolation.
Hope this helps.