As suggested in below link;
https://forums.juniper.net/t5/SRX-Services-Gateway/CoS-over-a-IPSec-tunnel/td-p/62727
if you want all esp traffic to have higher priority over other traffic , you can create an egress firewall filter on the external interface on which ipsec is egressing with a match term of "from protocol 50" and a then term to a forwarding class you want to assign esp traffic.
create your scheduler map and apply it on teh external interafce too.
Make sure you are running 15.1X49-D60 or above because CoS is supported over st0 after this version;