Is this what the engine manufacturer recommends? It is always better to cite a source or reference point.
There are two mixtures adjustments. A ground idle and an in-flight idle. The ground idle specifically runs richer on the ground to provide additional cooling.
I would recommend NOT leaning useless there is a problem fouling plugs. Follow the recommended guidance.
If you have any data to back up that statement, I would sure love to see it. Facts and data, not some myth about "fuel cooling".
Just on the topic of fuel cooling, lets examine this, fuel when it evaporates on your hand makes your hand feel a bit cooler, barely worth measuring and the latent heat transfer is very small. Now imagine how much effect that has with a very tiny volume of fuel inside a combustion chamber at 3-4000 degrees F. I think you would agree it is not even worth thinking about let alone measuring.
Fuel ONLY EVER adds heat to an engine. It never cools the engine.
Why you see the CHT change with fuel/air ratio changes is to do with the peak pressure generated and the the angle of that peak pressure from TDC. The gas expansion pressure at a greater Theta PP for a given fuel air ratio will generate a lower PP than one closer to TDC, and the higher the PP the higher the CHT, given the same rate of cooling air of course.
Now what does all this mean, well inside the cylinder, around the surfaces of the head, walls and piston crown there is a small boundary layer, just like the boundary layer on a wing. This layer is like a thermal blanket. The greater the layer the better, and when the PP reaches higher levels this layer is reduced. More heat transfer to the head, and your CHT is higher.
If fuel cools, and thus less of it means hotter, why does CHT start dropping again once past30-40dF ROP? The CHT is cooler at peak EGT than it is at 30-40f ROP.
So back to ground operations, the vast majority of the previous posts are correct and are optimal operations.
The engine manufacturers do not always issue optimum information, they are often trying to appeal to the lowest common denominator, and often shoot themselves in the foot doing so.
Blind faith is a dangerous thing.
Probably the best source of education is found here
http://www.vansairforce.com/community/showthread.php?t=93201
and here, and a thorough understanding of the graph in the original posters opening post is paramount.
http://www.vansairforce.com/community/showthread.php?t=75132
Cheers!