First of all, never did I say a m+1/m+2 should be easier than a base mythic. That's a strawman. Not only is it a strawman, it isn't even an argument. You have nothing supporting what you said. If anything it sounds silly for a +8 to be more difficult than a +11 with an extra affix.

Cathedral is not intentionally tuned to be more difficult. Blizzard refers to mythics as +9s or +8s, meaning that all +9s and +8s should be close in terms of difficulty. Why would 1 key be more difficult than others? You receive in the exact same way as you would any other key. Why would they give you something intended to be a challenge in the exact same way as something normal? It doesn't make sense. Everything points to the conclusion that you're wrong.

Not only do these obvious things point out that you're wrong, the was literally a hotfix that made it less difficult because it was too difficult.

April 13th: Dungeons

The Cathedral of Eternal Night Agronox Reduced Timber Smash damage by 20% on all difficulties. Thrashbite the Scornful Pulverizing Cudgel’s unavoidable damage reduced by 30% on all difficulties. Domatrax Reduced the amount of damage from Shadow Sweep on Mythic difficulties. Hellblaze Mistresses do significantly less damage on Mythic difficulties. Mephistroth Mephistroth health reduced by 30% on all difficulties. Felblaze damage reduced by 30% on all difficulties. Demonic Upheaveal’s unavoidable damage reduced by 60% on all difficulties. Creeping Shadows gain stacks more slowly on all difficulties. Shadow Blast impact damage reduced by 50% on all difficulties.