The system being added wont change anything unlike your op is asking for. Its not going to allign people any more via the news we have of it so far.
As per raid io that has been working on the system with blizzard the system is “vary similar” to the way raider ios system works.
As such no it is NOT related to performance in dungeon’s it is ONLY related to experience. Making it performance based would not be anywhere near similar so we know that’s not happening. Just like we know there is not going to be a leaver tracker as that again would be a massive difference. Unless blizzard is hiding even from even raid io there working with that is possible but not super likely. As such it will not align people together better than now it will be the same as now. People can buy a run and get the experience or they can earn it. The system will not show if x is going to do more dps or of x knows the route well. It will only show how many times x has done it, this might mean 50 runs of them afk as paid carrys run them by or it might mean 50 runs of them tanking needing to know every element.
Blizzard adding it to the game is blizzard showing how key the score system is and how integrated it is to the m+ game.
No there should not be a bonus for running lower score players through instances that would only be rewarding those that already sell paid carries. While players should be rewarded for running the correct level of content for them not for “boosting” lower score players. It most def should NOT reward exclusives “forcing” players to have to replay content they are past to get some reward. At best a token reward they can earn faster via doing the correct level content would make more sense. The game should not be encouraging boosts more than already is. For example giving augment runes on completion would be able the limit I could see “rewarding” players for doing lower keys by making more runes drop as you go to higher keys so you want to do the best key to can but helping a lower player is not a total waste. As such you are not feeling forced to do so such as exclusive rewards would do.
That being said a spectate option for m+ and raid would be nice.