How are you getting your packet information into ES ? If you have it go through Logstash at some point in your pipeline you could easily just configure another output to a more robust datastore if you're afraid of losing data. If it's just for backup purposes I'd probably just dump it in a compressed file on my SAN through the File output plugin.
PS : 50K packets per second hardly seems like a small office's activity, that's almost what we get on one of our datacenters for 7K users :smiley:
PPS : also, unless you specifically require to store a copy of each and every network packet, I think you should look into using Netflow/IPFIX instead, that'd probably make it easier on your ES cluster than having all those packets going into it ;)