Nice idea, I support it.
Though I'd argue that even the existing penalty on Iron Grip is too harsh, given you need 11 strength and a feat slot for it.
I'd make the penalty for holding firearms at most mirror the penalty for holding spears, so a 15% accuracy loss or so.
The other idea was to have a special component for shotguns that enables them to be used as one-handed, even without feats, that would have been nice especially since shotguns are so low on enhancements compared to other firearms.