0% savings seems a bit low for any kind of volume. Did you run SIS on the volume at all? "sis start -s /vol/<volumename>" ?
As soon as you get more free space by deduplication, you can increase the volume size again. The number of physically allocated blocks cannot exceed 2TB. So if you save, say, 50g by deduplication you can increase the size of the volume by 50g afterwards
Re: Increasing deduplicated volume size after the threshold limit