1080Ti versus 980Ti is about 70% in these tests, though I think it's fair to say the variance is stronger - principally because 980Ti is short of memory at 4K.
I presume that LOD will not help with decompression, since textures would be stored once for all MIPs in a block that needs to be decompressed, no matter the MIP level required to show on screen.
The reduced memory of the cheaper cards will also hurt, in terms of "scratchpad space while decompressing", but that effect should be reduced with the right kind of pipelining. But decompression workloads do tend to be bursty in nature, even at the most finely-grained level.
I think it's going to be a couple of years before we see AAA games making heavy usage of Direct Storage. So those GA106s will be dead anyway.