I don't think you will find this information in a public forum. The throughput is affected by many factors including AV and AS settings, use of TLS and encryption, DKIM signing, use of content filters, size of emails, etc. You will find guidance on how many users a model of the ESA can support, based on general assumptions. You may find some "unburdened" numbers which tell you how many emails an ESA could process if all it did was relay the mail, but that is not real world. I think you will need to speak to your Cisco rep or SE if you want a number of emails based on your particular setup.
