With SNAT Automap enabled Performance L4 VIP - How to know the real client IP address ?

The most common solution to this issue is to set up an X-Forwarded-For Header. Enabling this will tell the F5 to add in an HTTP Header that stores the original client IP address. There are a few ways to enable this on an F5, the most common being an iRule such as the one below:

when HTTP_REQUEST 
{ 
    HTTP::header insert X-Forwarded-For [IP::client_addr] 
}

or just enabling "Insert XFF" in your HTTP profile so that the F5 will do it for you. This article seems to be a perfect read for solving your issue.

If you have any more questions, I am sure I can help.