To start off, understand that, if using wheel weight metal, I won't save bullets that aren't uniformly frosted, and make it quite clear in my literature that I recommend the same to my customers. If shiny bullets are mandatory to a customer, especially with heavy large caliber bullets, Linotype will have to be used so casting can be done at lower temperature.
The very thin cross-section between cavities does get much hotter than the thicker portions of the mold, and will always be the first place to begin frosting, but the problem, if one wants to call it that is far less than with iron or steel molds, which under similar conditions cause large sinks in the bullet surfaces.
So, my recommendation is to get the mold hotter till the bullets frost uniformly and enjoy the precise bullets produced. If you would like to have you loaded ammo wear shiny bullets, twist the loaded rounds in a wool rag a couple turns and they will shine beautifully, using any reasonably hard alloy. This will make your loads look better but have no effect on accuracy.
