The cap was raised to 925 with the release of Nighthold Normal and Heroic so we can assume the same will apply to Tomb with the cap increasing tomorrow or Wednesday for EU.
6/13: New legendaries available now @ 940
6/20: N&H ToS. All legendaries drop @ 970. Also, legendary upgrade quest that week.
6/27: M ToS. New Mythic Keystone 11-15 rewards, and "M+ loot cap generally goes up when Mythic difficulty of the raid unlocks"
Does this mean 6/27? Does it increase both weeks? (940 tomorrow (heroic+15), then 955 mythic ToS week?)