I upgraded to Ultegra so of course I wanted to switch out the bottom bracket. I honestly can't say whether or not this improves performance over the old version but it certainly does not hinder it. I would have love to buy a Ceramic Speed or Chris King bottom bracket but for the price it's hard to beat this one.
If installed correctly you get the best of what Dura Ace offer, total performance in pedaling. Note well that the left and right bearing cups are not labeled LR, and are weighed the same so if you get them mixed up good luck sleeping at night. The orientation is printed on the inner core sleeve. One must assume that the packaging is vigorously QA from factory to the day you get it in your hands. 41A is the MTB version.
The bottom bracket fit perfectly in my BB86 frame. Weighs 54.7 grams on my scale with all the green grease Shimano loaded it up with. Total width about 90mm, diameter that enters frame 41mm, interior diameter 24mm (fits newer Shimano road spindles).