Originally Posted by zacster

Here's a serious answer. You will climb better, shift better, spin better.

I'm 15 pounds overweight and not in terribly great shape, but I can climb better than most people I ride with and I credit my very stiff, very light Kuota Kredo for it. It certainly is NOT the engine, the bike just goes up the hill.
You will shift better and more often, Ultegra is supposed to be a really nice group. I don't know first hand though as I have Chorus. Chorus shifts better than the one time I rode a 105 equipped bike.

You will spin better, again because Ultegra is a better group and will be smoother.

Does this add up to faster speed? Only on the days your legs are willing to push it harder to take advantage of the above.