I'm of the belief that tank should pull it because it's the job designed to be most effective at distributing enmity. So for me, the issue I have with a tank isn't as much as if they're being slow rather than can they successfully generate and hold aggro.

Tales of DF thread has a gallery of tanks who are fast on their feet and fail to hold aggro. Or slow on their feet and still fail to hold aggro. When it gets to that point, the tank is factually detrimental to the party and thus leaves the party with very little options:

-Attempt to educate tank on how to hold aggro and hope it sticks
-Get a replacement tank if the above fail
-Take control of the leading pace because tank is unable to hold aggro consistently.

Concerning the last point, it all falls to the type of dgn the party runs and how well dps can hold aggro off the healer. If they succeed, the healer can actually get the party through the instance and even in some cases, right through the final boss.

As such, in my perspective, this would be the only scenario where I would fully accept the party, myself included, to pull ahead of tank.