The X value should increase based on instance difficulty, completion time, and number of mobs defeated.
Just saying such Systems would always lead to the stronger getting stronger while the average players will fall more and more behind.
It would not be healthy for the game.
Also a side note: orkham was fairly hard with only DC gear. When you obtain the items from the new instance, it will always be far easier than without. Same was in rofl. New instances should always be designed to have the previous instance gear. Although I would love a challenge mode, that is not mandatory, even for tanks
Greetings