You can only win CL or cup if you have an extremely good team or no +2 level opponents. When you promote and top level doesn't you rise up in the cup, so for example if there is currently a cup for level 60-62 and you are 61 and top of the server doesn't promote, there is a good chance that the cup stays around 60-62 and you are then 62 and get worse opponents from low levels.
Top 8 is managable for an active player. My goal is always top 4, but I'm not right at top of the server yet, 6 more levels.
What level are you currently? It is worth saving some ressources until you are top of the server to be able to stay there, but if you are 6 levels away from that like me, it takes you around 18 more months if you always promote...