good news for everyone which uses SSL Offloading with Virtual Server.
FortiOS 5.2.8 is supporting new TLS 1.2 which was until now not the case and which is only the case under FortiOS 5.4
All devices which are able to use SSL Offloading (consult software matrix) have under FortiOS 5.2.8 this possibility.
# get system status | grep Version
Version: FortiGate-300D v5.2.8,build0727,160629 (GA)
# config firewall vip
(vip) # edit test
new entry 'test' added
(test) # set type server-load-balance
(test) # set server-type https
(test) # set ssl-min-version
ssl-3.0 SSL 3.0.
tls-1.0 TLS 1.0.
tls-1.1 TLS 1.1.
tls-1.2 TLS 1.2.
hope this helps