How does that work in your mind?
You can get 5 points of mainstat per point of Paragon above 800. That is a direct translation of power and toughness. I don’t see that as a less increase, but when taking in the whole of your character, assuming you have near max items with augments in place, high level gems, mainstat alone isn’t going to increase it much, but at say 1000 Paragon at time, thats a 5000 Mainstat addition that can’t be ignored.
So I would say its not so much diminished as it has less effect because you have maxed everything else out along the way and that’s all that’s left to boost your total capability.
Meaning that:
You were upgrading gear, gems, augments along with paragon at the same time till you hit a point where the gear, gems and augments are essentially done or maxed and all that’s left is the paragon.
Further, this can be clarified with a simple experiment. Take a character with 5800 paragon, but you run 10 rifts at a time with only say 1000 paragon worth of points added to your mainstat with those rifts being ran a level where you can complete them in say 10 minutes average. Run 10, add a 1000, run 10 add a 1000, all while keeping the same GR. Alternatively, you could increase the GR level to maintain the same time completion. I’m using a 10 minute run time, as when you are clearing at sub 3 minutes the changes aren’t as noticeable. The increase in capability should still be linear at this point assuming no other changes.
Anyway.
Just an idea.