If you're using 8.3 you should be running V28, unless you're testing it (and in that case the alert system would not matter).
Running v15 and a zpool v28 implementation makes no sense and it should be warned. The idea about v28 being slower does not justify that, this is just a flaw theory, in fact the zfs v28 implementation is slower, even if you're running a zpool v15.
Besides that, the GUI has some zfs screens that assumes you're running the latest pool version, like dedup and zle compression.
True, I'm running 8.3.0-Beta3 on a VM with a V15 pool. My real NAS machine is also V15 pool and running 8.0.4 but I'm hoping to upgrade it to 8.3.0-Release in the coming months.
I didn't realize V28 is the required pool standard for 8.3.0 and I thought it was always an option to convert your pools, not a requirement. When I say option, I mean FreeNAS would run normally with the V15 pool, not they way it currently runs which is functional but expecting V28. With that said, I will remove my ticket and wait to see if people have problems (hopefully not) during the upgrade to V28. I plan to backup all my data to a separate NAS but that will take considerable time, likely days as the old NAS is not a speed demon, before I make the change.
As for dedup, I saw the menu you are talking about, including the compression so I understand what you are saying. Too bad these also couldn't be disabled for a V15 pool. Dedup is fine for a serious business providing they have an enormous amount of RAM to support it unless that has changed too and requires not so much RAM. I myself would ever need dedup nor zle compression but I'm sure others will give it a go and appreciate it.
As a recommendation, maybe the alert window could be a bit stronger in stating V28 is the required pool version for this version of FreeNAS and pool conversion is required however it will operate in reduced capability until the upgrade to V28. Just a thought.