There already is an accuracy penalty when 'hip-firing' (or to be more specific, spread is reduced slightly when you use iron-sights, and also when you crouch).
It's quite subtle though, and I agree that some weapons are possibly a bit too easy to get headshots with without using the ironsights, so I wouldn't necessarily be against tweaking it a bit. Perhaps the Commando perk should get a bonus though, one of the more useful aspects of the perk is that you can quickly and reliably deal with trash without needing to use sights all the time.
Also, fix the third-person ironsight animations so we can see when our team-mates are aiming! The animations seem to exist for at least some of the weapons, they just aren't used most of the time.