Is there a Fortinet recommended version of the System Director?
This is not an official Fortinet recommendation, but for my Meru customers that still have some or all 802.11n AP's I use 7.0-10-0, and for my customers with all 802.11ac AP's I use 8.2-4-0.
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I have read through the release notes and don't see any pressing issues that would require an 802.11ac environment to upgrade from 8.1 to 8.2.4. Is there an page where Fortinet officially says it recommends going to a particular code level because of xyz?
We are working on coming up with an official recommendation, but the release notes are all we have for now. There isn't anything that specifically says go to xyz release, so you have to go through the new features list and/or the fixed and known issues lists in the release notes in order to determine the best release for your customer. You can always reach out to your local SWAT resource and they will help you figure out the best path forward.
As mentioned by Matt this will depend also on the actual installation.
For myself, then I prefer the 8.1-3-2 for all 11ac wave 1 AP's :)