Forget about whisking the sauce by hand. Put all of the sauce ingredients in a glass measuring cup or another deep container along with 1/4 cup of water and use an immersion blender to mix them. A few seconds of immersion blending produces a sauce that pours perfectly.
I like a little fresh grated ginger and chopped cilantro in my peanut sauce.
Biggest thing I find with soba is to make sure you tend to it while it is boiling and then rinse it well, otherwise it gets sticky and gummy. I rinse with cold water in a colander and then transfer the noodles into a bowl of cold water. At that point I scrub the noodles to remove the starch and drain back into the colander.