TBH it's hard to find answer to this question. The best one is Barshen Beasts, I've added SFX to slash attacks and enemy's shots and the game feels waay better.
On the contrary, there are opposite situations in which removing a feature made a huge difference in a positive way. I'm used to crouch or lying down feature from Contra series, however when making my own run 'n' gun, I couldn't draw and code proper crouching sprite so I removed this feature. The result was huge, it makes aiming downward easier and not conflicting with crouch mechanic.
Because of that, I don't add crouching mechanic to my run 'n' guns and I could make them quicker.