I understand that Soro, Tiagra, 105, Ultegra and Dura Ace in the Shimano lines are all good. I have ridden 105 and Ultegra. I don't really notice any difference in feel.

If you stuck a racer on a bike, on flat terrain, 105 should give him about 99.99% of the performance as Dura Ace. On long hill climbs, power to weight is what determines performance. Dura Ace is lighter than 105 so a rider may be about 5% faster on climbs on a top end bike because they are lighter.

As for durability, I do not think that anyone has gone out and tried. It takes a really long time to wear out components. In my experience, most components are good for 30,000+ miles which is a distance that most bikes never ever see.