This is something you're going to get a lot of differing answers on, because a lot of people feel it's common sense and intuitively obvious that you're graded compared to your mmr. However, the rioters involved in making the system have said that grading is absolute and does not depend on your mmr, so getting an S as a bronze requires a similar performance as getting an S as a challenger.
That said, the different groups are probably going to earn their 'performance' differently. High elo games tend to have far fewer deaths, so peoples overall kills tend to be lower, but their cs is way higher. Low elo tends to be a monkey death match with 5 kills/minute, so the bronze 5 player who is 30/2/20 at 20 minutes with 100 cs may get an S because of their crazy kda despite crap cs, but the challenger who is 3/0/2 at 20 minutes with 230 cs might also get an S because of their crazy cs (and if their scores were reversed, they'd still get S's). (Theres lots more to grades than cs and kda though, but just to give you an idea of how the grades might differ in practice despite having the same requirements.)
