Irule to modify timeouts ?

Use the iRule code inside ACCESS_SESSION_STARTED event not CLIENT_ACCEPTED:

when ACCESS_SESSION_STARTED {
    set user_ip [ACCESS::session data get "session.user.clientip"]
    if { not [class match $user_ip equal datagroup } {
        ACCESS::session data set session.max_session_timeout 200
    } else {
        ACCESS::session data set session.max_session_timeout 300            
    }        
}