I play online FPS with friends for fun. I don't care about leaderboards, but I know people that do and don't want to take them away from them.

You can't have accurate matchmaking and allow cheating. People cheat for a variety of reasons, at lot of cheaters are just online bullies that enjoy tormenting other players. In a low ELO lobbies, you would have cheaters that have top tier aim activated only if they lose too much, making the experience very inconsistent.

Top tier ELO would revolve around on how the server handle peeker advantage and which cheater as the fastest cheating software. It's an interesting technical challenge, but not a fun game. As soon as a non cheating player is in view of a cheating player, the non cheating player dies. That doesn't make for a fun game mechanic.