Kruse, here's the crux of it:
You are saying that you get to modify the Combat Familiar's attacks by the effects of the weapon that the owner is wielding.
You are NOT proving that the Combat Familiar's attacks should be modified by the weapon.
RULES AS WRITTEN state: The model with the familiar makes two attacks at S4 AP-. Done. Nothing about modified by current weapons or wargear.
Also remember that Codex always supersedes BRB.