Prior to the hotfix, this Cache contained 400 base (pre-Knowledge-modifier) Artifact Power. After the hotfix:
Mythic 2-3: 500 base Artifact Power
Mythic 4-6: 800 base Artifact Power
Mythic 7-9: 1000 base Artifact Power
Mythic 10+: 1200 base Artifact Power
At the same time, any additional (bonus) chests awarded for beating the Mythic Keystone timer by a very large amount will no longer contain any Artifact Power. They will still contain items of the appropriate power level, as before.