I installed this mod for my server after seeing some Youtube videos of it.
We noticed spawn rates where lower at y level 50 - 100. Then we learned why after digging down a reached y -440 before stopping.
The mob spawning algorithm uses the height of the block.

So where can I find a list of actual changes to generation provided by this mod?

The mod page explains everything. If you're going below y -64, that is caused by another mod.