IMHO... you always need to work with the sink FIRST. That's not to say that the chipset may not have problems, but 9 times out of 10, it is the sink. The reason being - chips are manufactured to much higher tolerances than heat sinks. While there may certainly be some concavity on chips, this effect is far more likely to be minimal in nature compared to the potential concavity or convexity of sinks.
I can see a couple of issues immediately form your picture. First, the heat sink is anodized aluminum. Going to a copper sink will provide better thermal transfer. Secondly, I do not recommend that your sinks be anodized. This is a form of corrosion protection that really isn't needed for a heat sink; in any case, aluminum will for a protective layer of Al2O5 immediately on contact with air. A sink that is 100% anodized is almost always an extruded aluminum shape - that is, not milled to exacting tolerances.
If it were me, I would focus on the following issues:
1) Replace with a copper heat sink if you can find an economical choice.
2) If not, remove the anodizing from the bottom of the heat sink.
3) Lap the sink to remove the possibility of not being flat.
4) Use a quality thermal compound. I personally prefer Arctic Silver 5, but there are some other good ones available.
5) If the above combination does not help, only THEN consider lapping your chipset. Thsi si obviously a drastic step and should be approached with caution.
Actually, I DO have a vendor in Denmark that resells my kits. Try www.webconsult.dk
and search for "lapping kits". Please feel free to email me with any questions that you might have.