Sounds very similar to WOW and its challenge mode dungeons, which had item level caps and leader boards for clear times, with things like vanity items and mounts as rewards for completing them with high scores. They were great, and actually required a decent level of skill and coordination to clear each dungeon fast enough for a gold rating while still meeting the quota on enemies killed.

Would be good to see something like that as an optional challenge, and to encourage people to do dungeons properly instead of just faceroll through to finish their daily roulette add quickly as possible.