A couple of years ago on a training ride with a group of friends one of the guys had a problem when his lightweight vest fell out of his jersey pocket and was sucked into his cassette, destroying his Dura Ace rear derailleur. We were on vacation away from home so he took his bike to the closest local bike shop. The shop didn't have a Dura Ace derailleur in stock so they had to substitute a 105 derailleur as a temporary measure so he could finish the rest of our week of training. A year later, I noticed that he still had the 105 derailleur on his bike, so I asked him why he hadn't replaced it. He told me that as far as he could tell the 105 derailleur worked every bit as well as the Dura Ace so he couldn't justify replacing it for no functional gain. Shifters make a bigger difference in performance than derailleurs