In the new Patch 8.3 Visions of N'Zoth Battle for Azeroth (BFA) you get more loot starting above mythic +15 keystone. The ground rules are:

  • You get only 2 items for any dungeon done not-it-time as guarantee until you reach +16 keystone level;
  • Each Keystone level starting at +16 will add an additive +40% chance for an additional piece of loot to drop;
  • At Keystone +18, you'll get an additional guaranteed piece, with a 20% chance of another item and at Keystone +20, you'll get two additional pieces;
  • This stacks with the additional guaranteed single piece of loot you get for completing the dungeon within the time limit. 

The table below shows Number of Items you can expect in the end-of-dungeon chest depending on the Key Level you run with your team:

M+ Level Items Dropped

In case of Beating the Timer

% Chance for an Additional Item
+15 2 3 -
+16 2 3 40%
+17 2 3 80%
+18 3 4 20%
+19 3 4 60%
+20 4 5 -

This way, you want to look at this table when you are gearing up your character. It's always a personal decision whether it's worth pushing high keys to receive additional loot or just focus on +14 keys which reward the same item level gear. The best way to look at this delema is to analyze your current gear and game skill. If it's pretty easy for you to complete the +18 dungeon in-time then you should go for it and enjoy the 4 dropped items. In case +18 in-time seems hard for now, farming +14 keys and doing them in-time is a better option as they will get you 3 items of the same item level.