Thanks for the info, I'd read that doc along with quite a few others on this topic. This system has a single use case which is VMware datastores. I had generally hoped to be able to setup volumes/luns and basically leave it alone. The volumes size is probably 95% of the aggregate, and there is no room for it to expand if needed so not sure the autogrow would really do much here.
It sounds like the only thing I can do is to over provision, and just keep a close watch on the used volume space before it becomes a problem. I had a support ticket on this and asked repeatedly for someone to just please verify that, yes you had to overprovision luns to actually gain the benefit of dedupe but could never get anyone to actually say that. It has a been a rather frustrating experience for what I feel like is a pretty straightforward and simple use case. Thanks for verifying what seems pretty obvious, but is kind of a hassle to work with imo.