The only major house rule I have done in CoC is to combine skills. Sneak + Hide become "Stealth," and so on. I also try to "gumshoe" the game by making sure the core/key clues cannot be missed by a dice roll.
As far as reducing the power of damage... well, remember that CoC is lethal and dynamic, and I would hesitate to reduce the capabilities of their weapons. Even if they are able to eviscerate a mythos creature, most creatures can do the same to them with one good attack. And if the characters were shot at with full auto and hit, they probably shouldn't live either.