Its quite easy to reduce the armour setting with the slider when placing the unit in the mission or script it. But the knock on effect is that it leaves it more vunerable to small arms fire.
I think the 'Strela' and 'Stinger' missiles in OFP dont have the destructive power it ought to have. Look at the Afghan war or Chechen war were they have been used to great effect.
I have shot down lots of helicopters in MP but i think that is due to the mission designer deliberately reducing the armour values of some units to make it easier.
Another example: with Tanks, many times you hear people bitchin in a MP game that it only took one sabot to kill them in a M1A1, but they shot 3 sabot at a T-80, maybe its a MP thing, or the mission designer orchestrated it to happen like that.
My main request though is for the AA misiles to get that noise like a high pitched tone, signifying the missile is locked on.