Various experiments by others have addressed adding fillers such as silicone and wax to HP cavities to improve expansion with sometimes impressive results. Anyone that's PC cast HP invariably ends up with a few boolits having an excess of powder in the cavity. This begs the question of just what happens?
The following are all standard for caliber pistol velocity loads ranging from 900-1050fps cast from 20:1, fired into ballistics gel covered with x2 layers of a pink polo T-shirt. The pink fiber makes for nice contrast when assessing any plugging of the HP.
The boolit on the far left of each group intentionally had the HP cavity completely filled with PC powder before baking and looked like a flat nose boolit with a slight dimple after they cured. The other 4-5 projectiles to the right were coated and had excess powder tapped off as I would normally coat/load/shoot.
Here's what I found:
NOE 358-135 9mm (128gr):
Mihec 360-640 9mm (145gr):
NOE 403-200 WFN .40 S&W (180gr):
As can be seen, a varying degree PC plugging does occur and appears to have a more detrimental effect at lower velocities (heavier boolits), and larger cavities seem more susceptible to plugging than smaller ones. PC plugs in the nose do not behave like a ballistic tip or other expansion enhancer. My take away is that if there is a small amount of powder left in your HP cavities after coating, it's not enough to matter.