FortiAuth 5.3.1 Upgrade

Is anyone running on 5.3.1? 

My Fortigate's are remaining on 5.6.2 based on the chatter around SSL VPN instabilities and I would be interested to know if anyone is using FortiAuth 5.3.1 with a pre 5.6.4 release OS (although this is not recommended based on the release notes). 

Out of interest I wonder if anyone from Fortinet can comment on how constrained the FortiAuthenticator support for FortiOs is - i.e. the notes for 5.3.1 specify 5.6.4 and 5.6.4 only. There are a couple of things in FortiAuth 5.3.1 that I would like but I cannot risk SSL VPN instability as this is one of the main features I am using. 

Thanks

Mike