There are a lot of factors that can play into how good your score is.
If you just leave a game the moment things are going south, you'll maintain a pretty impressive K/D/R. Not exactly an optimal play-pattern to enforce, so a lot of abuse cases would need to be thought out.
If you add assists into the mix, it's fairly difficult to come up with situations you actually did well but it wasn't reflected in your score.
If you manage to acquire 2 assists for every death you were probably going even.
Tracking that would be nifty to bump up/down players that really don't belong in their ranked division, as well as tracking and identifying smurf accounts to just place them against other smurfs rather than ruin the experience for newer players.
Accedents happen and you can certainly do amazing against your peers in one-off stuff, so figuring out a system to account for that would also be important, but overall yeah - individual performance would be a cool thing to track if it was done well.