Anyway, SR 14 gives the full plate item a 2000+ value. You raise it to 16 and it's in the 5k range, something that would drop rarely. Make it SR 18 and it's in the 9k range and it will never drop. A -2 influence would hardly drop the price of an SR 18 full plate. Meaning it has to be something a bit more nasty than just influence.
If we follow Jerv's suggestion and make a AC -2 drop + the influence, the SR 18 full plate drops to 6k. Now being nasty and considering it's a witch hunting armor, meant against mages, not an armor you'd use for front line combat, but against witches if you add 25% dmg vulnerability against slashing, piercing and blungeoning... the armor drops to 4259. Right in the range of dropable things.
And if we do it like you suggest, Helaman, armor is at 7777. It will never drop.