Yeah but only a few iconics can have mods, and most of them go directly to the stash (even half of them don't even show up on the wall )??? The Moron Labe has two mod slots at tier 2 . The Lexington you win at the shooting range has at least one mod slot depending on what tier it is.
But for the non-lethal silenced gun gameplay, most of the weapons can't be modded. Jackie's guns, Death&Taxes, Her Majesty to name a few.