I have heard that in OGL and in DX9
D3DPOOL_MANAGED mipmap layers are pulled to the GPU memory on demand. As in If I pull from layer 4 of a mipmap, that layer and all smaller layers are pulled into the GPU memory, but larger mipmap layers are not.
I have tried to test this by creating a scene which requires more textures than the GPU has room for, but never getting close enough to any object such that it's larger mipmaps are required.
In OGL I don't hit a performance cliff, and as best I can tell with gDebugger I don't fill the GPU memory.
D3DPOOL_DEFAULT I actually get a DX9 error cause I run out of GPU memory space.
At such a high level of abstraction I can't conclusively say what's happening. But I was hoping someone here could confirm or deny the partial mipmap rumor I've heard, as that would make my tests make sense.