Sry,but KDA is one of important things to take a decent grade,and your 17 minute game far not the best argument against it.Grade system gives you grades based on overall stats on this champion IN THIS VERY EXACT GAME LENGH,so it obviousely means that the longer game go > better kda,farm,etc you need to have. So,according to league graphs http://www.leagueofgraphs.com/img/infographics/patch-7-2.jpg?t=412695,as you can see games with 15 min duration almost never happen's,so 17 game is has kinda around 1% by logical thinking.So in other words.You have almost no one to compare your gamestats with which leads your grade to almost always S if game ends before 20 min.AND having no death means perfect KDA so he's definitely not bad in here.I have 9 lvl 7 mastery champions and that's the thing i figured out - if you're ending game before 20 > free token.Hope i explained it well enough
P.S. Forgive my bad english,might have some grammar mistakes :(