Thunderlord's/Precision combo might be a tad strong currently
...when, out of 18 champion classified as ADC, 13 have the combo of Thunderlord's Decree and Precision in their respective highest winrate masteries.
http://champion.gg/statistics/#?sortBy=general.overallPosition&order=ascend&roleSort=ADC
And yes, that includes champions like Vayne, Tristana and Kalista, who are primarily reliant on auto-attacks.
And honestly, I think the part that is imbalanced is primarily not the keystone, but the penetration mastery under it (which got buffed to high heavens on this patch).
From its numbers, Precision is only inferior compared to Battering Blows and Piercing Thoughts against full-on tanks. Not just bruisers with 1-2 defensive items.
For reference, the breakeven point is 79-200 Armor (after a potential 
item) and 47-120 MR (again, after a potential 
). On a 25-80 base Armor tank, that is 123-298 Armor before the (PBE buffed to 45% ArPen) LW items. For MR, 72-185 MR before Void Staff.
218 bonus Armor on a tank is 208 before the resistance bonus mastery. 201 before flat Armor runes are added. For 200 armor from items, you'd more than Frozen Heart and Thornmail - and that combination lacks any health.
185 MR on a tank is 176 before resistance bonus mastery. 122 without base MR. 110 without flat MR runes. Again, 110 MR is more than Merc Treads and either Spirit Visage or Banshee's Veil combined.
