Originally Posted by
Caliper
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.
While the narrow plates may mean less movement, they do make set up more difficult and limit the range of gears that can be used without rubbing. I would doubt that the tolerances are that much better. The lower end components don’t shift any worse, they just work better over a wider range of gears.
On the other hand, SRAM front derailers don’t have the same narrow and sculpted plates. Their high end derailers offer the same weight savings but they work and are easier to set up than the high end Shimano front derailers.