Originally Posted by
cyccommute
It’s not that they are just lighter. There is a lot of difference in the dimensions on the plates between the higher end models and the lower end models that has an effect on how they work. Deore and Tiagra, for example, have more space between the inner and outer plates that allow for easier set up and wider range. The narrower plates on the more expensive derailers don’t make them work any better even when paired with the same level of components.
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I beg to differ. Narrower derailleur plates mean you can shift with less movement of the cage. That translates to some combination of less travel and/or lighter effort at the shift lever. The kind of stuff people talk about when they spend more on a groupset than an entire bike with lower range components costs. To get the derailleur to shift this setup well, they probably also hold tighter tolerances in the shifter and on the chainrings/cassette and rely on clean cables and following all the setup instructions. You'll also notice that the highest tier of components also tends to have a slightly narrower gearing range that probably helps with shift quality as well. The top end of bike components have never been meant to work perfectly with anything, they have been meant to work on flagship bikes bought by riders who can justify & afford to spend the price of a nice used car on a bicycle.