[Theslaking]

Silicone will definitely cause a noticable amount of additional heat as any generated won't pass through as easily. Silicone is a high heat barrier.

Now I can't imagine the amount of heat generated from a speaker in a door being an issue.

Factory speakers don't usually end up damaged from water but most of us get rid of factory mounting or add speakers with more depth in doors which creates a need for more shielding.