A/A still works by load balancing UTM (AV/IPS) stuff to the second Fortigate.
Load-Balance all trys to load balance even single TCP sessions to the secondary Fortigate.
The overhead needed for that (New TCP SYN is coming to fgt master, replicate that session over HA link to the secondary FGT ...) is in general more expensive than the acceleration you may gain.
There may be some corner cases where load balance all makes sense (lots of elephant flows?) but in general: do not do it.
If you thought about using HA as "twice the firewalls, twice the performance" you will have a hard time.
There was a concept of independent firewalls(clusters) which synchronize their sessions, but i can't find the paper.