Ultegra is a little lighter, a little smoother and holds tune for a little longer, but I don't know whether it is necessary to achieve that by upgrading to full ultegra. Perhaps you might consider upgrading just a few of the components to ultegra, such as the rear derailleur, cluster and chain. I found that by changing these parts alone (keeping everything else 105) is all that is necessary to match the performance of a full ultegra groupset.