The reason that you would use source address translation is to cause the response to return via the LTM, and not directly to the client.
You could disable source address translation if your pool members are configured to route traffic via the LTM, eg, if their default route points to the LTM.
Alternatively, for HTTP traffic only, you could have the LTM insert a X-Forwarded-For header with the client's original IP address.
F5 provides an ISAPI Plug-in, which you can download from the LTM's default page (click on the red f5 ball to get back to it) if you wish the client IP to be written to the IIS logs. See SOL4816 for more information on this.