For completion, here is the eventual response I recieved,
"After we have compared the autosupports from before and after the expansion:
- /aggr0/plex0/rg0 expanded from 11 to 15 disks
- volumes do not have space guarantee,
- used data on volumes has gone down as well.
it turns out that right now there would be no immediate need to use reallocation as there would be (1) enough space on disks themselves and (2) enough disks have been added to the raid group to avoid 'hot disk'.
The only scenario when you would need reallocation is when there are performance issues due to deleting large amount of data (causing large fragmentation). If such a problem ever occurs in the future, please open a performance issue (Cat 1 - Performance) and we will assist you accordingly."
Thanks for the input on this thread!