The grading system isn't very good, in my non-professional opinion.
Alrighty-o, so to start off, this is all strictly my opinion, so don't get your cloth armors in a bunch.
I have an issue with the grading system, but before I delve into the reason why, I'll let these things be known: I know that grading doesn't only count KDA; I know that it compares your score to other people in your region.
Now the problem with this grading system is flawed, due to the whole "you didn't do as well as this other guy!" system.
Seriously! 8/2/21 as Alistar on ARAM and I get an A just because I let others farm! Or maybe I didn't place enough wards ¯_( ͡° ͜ʖ ͡°)_/¯
Why do I need to aspire to be "as good" as someone else? If I perform well, help my ADC, get 60%+ kill participation, spam wards and walk away with 0-2 deaths, why do I get an A+ just because I didn't get 150 CS from under my ADC's nose? I buy Ancient Coin for a reason! I upgrade it FOR A REASON.
Am I lead to believe that I should steal my ADCs farm so I can get a higher score? That's pretty homosexual, if you ask me!
Grading shouldn't be based off of how someone performed in comparison with someone else, it should be based off the actual performance of THAT game.
Now, if I were in charge of Rito, I'd make the following changes to the grading system: The CS of supports is irrelevant in grading, leave that for non-supports. I really rather not take the wave and tell my flaming adc "Sorry, broski, Rito says I'm good if I have CS :/ "; No grading based off of comparisons. It's never fun getting a lower grade when you did exceptionally well, just because some guy 5 blocks away got 2 more CS and places 4 more wards than you.
That's about it.
Oh and remove Rengar, thanks.