Something to note:
Last Whisper now costs 1300g (Pickaxe+425g), gives 25 AD, and gives 30% bonus penetration.
Last Whisper now has two upgrade options:
One for dealing with tanks: Giving extra % phys damage against champions with more health than you.
The other for dealing with healers, applying Grievous Wounds on hit for 5 seconds (GW reduces healing by 40% now)
In either case: Both cost 2700g each (includes component costs), give 40 AD, and 40% Bonus Armor Penetration.
In other words: If they just have Last Whisper, Thresh's defense was straight buffed.
If they finished one of the two upgrades, it's a buff if Thresh's bonus armor is less than 112, and a nerf if it's greater. (Still, Thresh is fairly problematic in that he does too many things and doesn't really pay much for it.)
That said, it's a fairly minor nerf. At 300 bonus armor, he's only lost about 10 armor, and still has 196 armor (down from 205~)