IIS logs show LB IP address for all requests

It appears you are using the snat function, which translates the client IP to the F5 IP. You will need to pass the client IP in a header, done in the HTTP profile by enabling the Insert XForwarded For setting, or in an irule. You will need to extract this information on the server side, I think there are examples how to do this for IIS on this forum if you search for it.