Uh no.
https://us.battle.net/forums/en/wow/topic/20751995042
I didn't think the source was necessary as I thought it pretty common place, but I regularly run M+ and 11 definitely doesn't have the same rewards as 10
10 drops base 865, and 11 drops base 870. From that point it continues to be 870. 865 was the cap during EN, and all the rewards past 10 gave you base 865 loot, and obviously you got 880 cache loot, as the formula of +15 from the base had held steady.
I mean go check it out yourself, there is a reason why I, and others like to farm +9 keys. If you +3 them you get 2 pieces of 870 loot from the 11 chest, 2 pieces of 865 loot from the 10 chest and 2 pieces of 860 gear from the 9 chest.
They would (at least if I designed the system) change 12 to give 875, and 13 to give 880 because it fits the model of the above examples. 14 in such an example would be the tier in which you get the maximum cache reward (+15 from base 880), as it lines up perfectly with heroic Gul'dan iLvL, and the first 3 bosses in Mythic EN.
Currently 10 gives 865 base, 11 and beyond gives 870 base. 10 and 11 both give 880 cache loot, while 12 is rewards 885 cache loot (which is the same iLvL as Mythic ToV prior to the +5 bump in iLvLs).
Again, I'm not saying that this is what it's going to be, I'm just saying based on how they have approached the system in the past, that it's likely they would head this direction. They don't really have room to stretch rewards out past 14 right now, as it would seem silly to put further rewards on 15 as it's suppose to be an achievement by itself. They also could abandon their philosophy on M+ and not change it at all, which I think would make people pretty unhappy (although for some it would make them pretty happy as they wouldn't need to do it). Essentially the system, at least how it was portrayed was to give you one piece of Mythic iLvL raid gear once a week, and the base rewards giving something around what you would get for clearing heroic. To achieve that design goal (unless they want to tone it back some), we would need base 880 gear, and cache 895 gear. Would it be the end of the world if it was 875 and 890? IDK, but it would be a departure from what we are use too.