While I approve of the general idea, I can see a few problems with the specifics of your suggestion.

1. The fastest time is not something most players will or should care about. More likely than not it will be set by a group of hardcore raiders with a static, and will be an unrealistic goal for the average player.

2. It would measure the performance of the group, not the individual. A weak player would be boosted by the stronger members of the group and be left with an overinflated opinion of themselves.

3. The players who don't care about improving themselves will ignore this and will continue to join parties they have no business being in.

To be successful, the performance measurement system will need to measure individual and not group performance. This can be achieved in group content by measuring DPS instead of time. A solo proving ground, should one be implemented, will need to contain mechanics and not just a passive striking dummy. The performance measurement system will also need to provide a realistic and relevant goal. You don't start training for sports by comparing yourself against the world champion. A good point of comparison is the DPS required to beat the fight before enrage. Mechanics training would not be amiss either. And finally, to really solve the problem of players joining content they are not qualified to be in, the system can't be optional. As long as it's possible to ignore, there will be players who think their right to have fun trumps everyone else's.