The offhand attack speed modifier on the guns affects the melee attack speed of your mainhand. This leads to some goofy interactions with other mods and items, like being able to swing swords and axes as fast as you can click. AFAIK, there isn't any benefit to having an attack speed modifiers on the guns, as they aren't meant to be swung in the first place.

I think that the NBT tags for attack speed, reach etc. should probably just be removed, for better compatibility, at least from the offhand.