I did have a go last year when I noticed the issue (and just tried it again now by altering the 'ExitTimeOut' to 10 seconds from its default 65 seconds), but it still didn't change the shutdown duration for me. I think this is because although the plist file is named with the process 'esets_daemon', in the file it actually is for a different process, esets_ctl.
It's probably best to wait until ESET releases the major ECS/ECSP update around El Capitan's release soon.
Works fine here, I have it set to 1 second and shutdowns are now instant.
You do have to have gone through one reboot cycle for the new value to be picked up unless you unload/load the daemon.