I would worry about the frustration brought upon sending higher level people into lower dungeons. It was bad leveling, can you imagine a CM speed runner in Halatali? Maybe it wouldn't be a big deal, but they will think of it as SE forcing them to do lowby dungeons and they will not be happy.

My idea I think would be a little better. Soft cap, once you hit 300 you implement diminishing returns. So instead of 40 tomes, you get 30. Then 20, then 10. Probably leave it at 10 until it resets.

Sane people wouldn't grind so much after 300, but they would still get something if a FC wanted to run a dungeon and asked you to help. It would also encourage people to log in and not just wait for the reset. This in conjuction with the random DF reward system in 2.1 should help alleviate some of the problems we currently face.

That's my opinion, and the method I would go with if I were SE.