This is not about taller mountains, since from what I understand, that would make the game really heavy. So here is my humble suggestion:
Would this be any more demanding than vanilla? I really don't know how it works.
Opinion:
I believe that this would make mountains more interesting than just a wall of rock. At the same time they wouldn't be that easy to overcome using nothing but two stacks of ladders. They would be a true Journey.
This is an example I made:
It is139 blocks high, about as much as a vanilla mountain.
Mountain.mp4
Please tell me what you think!