Another thing I want to add. The players in those tournaments are much better individually and as a team than most of us here. It's unfair to nerf a champion based on what the top .0001% of players do, because it's more of the player being really awesome, who can utilize that champion with 4 other players with extreme coordination, that most of us will NEVER see. It gives everyone else a really high bar to shoot for, and to compare our own play to.
Am I saying Diana and Rengar don't need nerfs? No. What I am saying is you are basing your balancing on something that the .0001% of players are capable of doing, and the other part of the population tries to emulate that (to little success most of the time). What I am saying is that you need to be much more mindful of the rest of your playerbase, take them into MORE consideration than you might give, and then act. It appears to this player, that you just take what goes on in a tournament, and nerf things based on that.