My apologies on the quartile analysis! Your numbers look correct. However, there are some things I take issue with here:

The whole team does not have to perform well for you to parse well. It can help, of course. But in some cases, the whole team doing well actually hurts you because it can get you a worse kill time. I've had many parses this tier where I get a 99 while the rest of the team is doing blues and greens because they delayed the kill time to the end of burst, so that isn't necessarily true. You can still do well without a whole team.

If you are doing a purely statistical analysis, then yes, the middle should matter most because the middle has the most people in it. But you also have to consider whether or not the middle is actually engaging with the entire class. A big part of this thread has been an argument about whether or not low-end players are using the tools at their disposal. If they are not using the tools, how can the balance of these tools matter to them? The reason I say balance matters more at the higher end is because the higher end is where disparity in performance actually starts to matter.

Think about fighting games for a second. The "best character in the game" for a midcore player and a hardcore player will be very different. A brand new player might find the "best character in the game" useless because they are hard to play. But they are still the best character in the game, regardless, and should be balanced as such. Midcore player's concerns should not be ignored, but when players are not using the tools in the game, how can their opinions o the balance of these tools be said to matter as much as the players that are? It's a tricky balancing act, but you can't ignore the top anymore than you can ignore the bottom.